Add proxy support for the editor

* Adds proxy support for `HTTPRequest`.
* Adds `network/http_proxy/{host,port}` editor settings.
    * Labeled as "HTTP Proxy" and it will be used for both HTTP and
      HTTPS requests. This is the same convention as seen in Android
      Studio's proxy settings.
* Makes Asset Library and Export Template Manager use proxy according to
  the editor settings.

doc/classes/HTTPRequest.xml

@@ -208,6 +208,24 @@
 				Returns [constant OK] if request is successfully created. (Does not imply that the server has responded), [constant ERR_UNCONFIGURED] if not in the tree, [constant ERR_BUSY] if still processing previous request, [constant ERR_INVALID_PARAMETER] if given string is not a valid URL format, or [constant ERR_CANT_CONNECT] if not using thread and the [HTTPClient] cannot connect to host.
 			</description>
 		</method>
+		<method name="set_http_proxy">
+			<return type="void" />
+			<argument index="0" name="host" type="String" />
+			<argument index="1" name="port" type="int" />
+			<description>
+				Sets the proxy server for HTTP requests.
+				The proxy server is unset if [code]host[/code] is empty or [code]port[/code] is -1.
+			</description>
+		</method>
+		<method name="set_https_proxy">
+			<return type="void" />
+			<argument index="0" name="host" type="String" />
+			<argument index="1" name="port" type="int" />
+			<description>
+				Sets the proxy server for HTTPS requests.
+				The proxy server is unset if [code]host[/code] is empty or [code]port[/code] is -1.
+			</description>
+		</method>
 	</methods>
 	<members>
 		<member name="accept_gzip" type="bool" setter="set_accept_gzip" getter="is_accepting_gzip" default="true">

editor/export_template_manager.cpp

@@ -147,6 +147,11 @@ void ExportTemplateManager::_download_template(const String &p_url, bool p_skip_
 	download_templates->set_download_file(EditorPaths::get_singleton()->get_cache_dir().plus_file("tmp_templates.tpz"));
 	download_templates->set_use_threads(true);
 
+	const String proxy_host = EDITOR_DEF("network/http_proxy/host", "");
+	const int proxy_port = EDITOR_DEF("network/http_proxy/port", -1);
+	download_templates->set_http_proxy(proxy_host, proxy_port);
+	download_templates->set_https_proxy(proxy_host, proxy_port);
+
 	Error err = download_templates->request(p_url);
 	if (err != OK) {
 		_set_current_progress_status(TTR("Error requesting URL:") + " " + p_url, true);

editor/plugins/asset_library_editor_plugin.cpp

@@ -39,6 +39,15 @@
 #include "editor/editor_settings.h"
 #include "editor/project_settings_editor.h"
 
+static inline void setup_http_request(HTTPRequest *request) {
+	request->set_use_threads(EDITOR_DEF("asset_library/use_threads", true));
+
+	const String proxy_host = EDITOR_DEF("network/http_proxy/host", "");
+	const int proxy_port = EDITOR_DEF("network/http_proxy/port", -1);
+	request->set_http_proxy(proxy_host, proxy_port);
+	request->set_https_proxy(proxy_host, proxy_port);
+}
+
 void EditorAssetLibraryItem::configure(const String &p_title, int p_asset_id, const String &p_category, int p_category_id, const String &p_author, int p_author_id, const String &p_cost) {
 	title->set_text(p_title);
 	asset_id = p_asset_id;
@@ -533,7 +542,7 @@ EditorAssetLibraryItemDownload::EditorAssetLibraryItemDownload() {
 	download = memnew(HTTPRequest);
 	add_child(download);
 	download->connect("request_completed", callable_mp(this, &EditorAssetLibraryItemDownload::_http_download_completed));
-	download->set_use_threads(EDITOR_DEF("asset_library/use_threads", true));
+	setup_http_request(download);
 
 	download_error = memnew(AcceptDialog);
 	add_child(download_error);
@@ -868,7 +877,7 @@ void EditorAssetLibrary::_request_image(ObjectID p_for, String p_image_url, Imag
 	iq.image_index = p_image_index;
 	iq.image_type = p_type;
 	iq.request = memnew(HTTPRequest);
-	iq.request->set_use_threads(EDITOR_DEF("asset_library/use_threads", true));
+	setup_http_request(iq.request);
 
 	iq.target = p_for;
 	iq.queue_id = ++last_queue_id;
@@ -1475,7 +1484,7 @@ EditorAssetLibrary::EditorAssetLibrary(bool p_templates_only) {
 
 	request = memnew(HTTPRequest);
 	add_child(request);
-	request->set_use_threads(EDITOR_DEF("asset_library/use_threads", true));
+	setup_http_request(request);
 	request->connect("request_completed", callable_mp(this, &EditorAssetLibrary::_http_request_completed));
 
 	last_queue_id = 0;

scene/main/http_request.cpp

@@ -554,6 +554,14 @@ int HTTPRequest::get_body_size() const {
 	return body_len;
 }
 
+void HTTPRequest::set_http_proxy(const String &p_host, int p_port) {
+	client->set_http_proxy(p_host, p_port);
+}
+
+void HTTPRequest::set_https_proxy(const String &p_host, int p_port) {
+	client->set_https_proxy(p_host, p_port);
+}
+
 void HTTPRequest::set_timeout(int p_timeout) {
 	ERR_FAIL_COND(p_timeout < 0);
 	timeout = p_timeout;
@@ -602,6 +610,9 @@ void HTTPRequest::_bind_methods() {
 	ClassDB::bind_method(D_METHOD("set_download_chunk_size", "chunk_size"), &HTTPRequest::set_download_chunk_size);
 	ClassDB::bind_method(D_METHOD("get_download_chunk_size"), &HTTPRequest::get_download_chunk_size);
 
+	ClassDB::bind_method(D_METHOD("set_http_proxy", "host", "port"), &HTTPRequest::set_http_proxy);
+	ClassDB::bind_method(D_METHOD("set_https_proxy", "host", "port"), &HTTPRequest::set_https_proxy);
+
 	ADD_PROPERTY(PropertyInfo(Variant::STRING, "download_file", PROPERTY_HINT_FILE), "set_download_file", "get_download_file");
 	ADD_PROPERTY(PropertyInfo(Variant::INT, "download_chunk_size", PROPERTY_HINT_RANGE, "256,16777216"), "set_download_chunk_size", "get_download_chunk_size");
 	ADD_PROPERTY(PropertyInfo(Variant::BOOL, "use_threads"), "set_use_threads", "is_using_threads");

scene/main/http_request.h

@@ -154,6 +154,9 @@ public:
 	int get_downloaded_bytes() const;
 	int get_body_size() const;
 
+	void set_http_proxy(const String &p_host, int p_port);
+	void set_https_proxy(const String &p_host, int p_port);
+
 	HTTPRequest();
 	~HTTPRequest();
 };