Captions: Add ability to use Innertube's transcripts API (#4001)

src/invidious/yt_backend/youtube_api.cr

@@ -557,6 +557,30 @@ module YoutubeAPI
     return self._post_json("/youtubei/v1/search", data, client_config)
   end
 
+  ####################################################################
+  # get_transcript(params, client_config?)
+  #
+  # Requests the youtubei/v1/get_transcript endpoint with the required headers
+  # and POST data in order to get a JSON reply.
+  #
+  # The requested data is a specially encoded protobuf string that denotes the specific language requested.
+  #
+  # An optional ClientConfig parameter can be passed, too (see
+  # `struct ClientConfig` above for more details).
+  #
+
+  def get_transcript(
+    params : String,
+    client_config : ClientConfig | Nil = nil
+  ) : Hash(String, JSON::Any)
+    data = {
+      "context" => self.make_context(client_config),
+      "params"  => params,
+    }
+
+    return self._post_json("/youtubei/v1/get_transcript", data, client_config)
+  end
+
   ####################################################################
   # _post_json(endpoint, data, client_config?)
   #