Add logic to fetch transcripts from label

Although available this method should be discouraged as it requires
an extra request to YouTube to get caption data in order to
map label -> language code and auto-generated status, which are needed
to fetch transcripts.

src/invidious/routes/api/v1/videos.cr

@@ -421,10 +421,11 @@ module Invidious::Routes::API::V1::Videos
 
     id = env.params.url["id"]
     lang = env.params.query["lang"]?
+    label = env.params.query["label"]?
     auto_generated = env.params.query["autogen"]? ? true : false
 
     # Return all available transcript options when none is given
-    if !lang
+    if !label && !lang
       begin
         video = get_video(id)
       rescue ex : NotFoundException
@@ -462,6 +463,26 @@ module Invidious::Routes::API::V1::Videos
       return response
     end
 
+    # If lang is not given then we attempt to fetch
+    # the transcript through the given label
+    if lang.nil?
+      begin
+        video = get_video(id)
+      rescue ex : NotFoundException
+        return error_json(404, ex)
+      rescue ex
+        return error_json(500, ex)
+      end
+
+      target_transcript = video.captions.select(&.name.== label)
+      if target_transcript.empty?
+        return error_json(404, NotFoundException.new("Requested transcript does not exist"))
+      else
+        target_transcript = target_transcript[0]
+        lang, auto_generated = target_transcript.language_code, target_transcript.auto_generated
+      end
+    end
+
     params = Invidious::Videos::Transcript.generate_param(id, lang, auto_generated)
 
     begin