Model: Move InterfaceConfiguration.publicKey to Curve25519.swift

The code for public key calculation need not be shared with the extension

WireGuard/Shared/Model/Configuration.swift

@@ -21,10 +21,6 @@ struct InterfaceConfiguration: Codable {
     var mtu: UInt16?
     var dns: [DNSServer] = []
 
-    var publicKey: Data {
-        return Curve25519.generatePublicKey(fromPrivateKey: privateKey)
-    }
-
     init(name: String, privateKey: Data) {
         self.name = name
         self.privateKey = privateKey

WireGuard/WireGuard/Crypto/Curve25519.swift

@@ -25,3 +25,9 @@ struct Curve25519 {
         return publicKey
     }
 }
+
+extension InterfaceConfiguration {
+    var publicKey: Data {
+        return Curve25519.generatePublicKey(fromPrivateKey: privateKey)
+    }
+}