Add API endpoint for fetching transcripts from YouTube (#4788)

src/invidious/videos/transcript.cr

@@ -122,5 +122,40 @@ module Invidious::Videos
 
       return vtt
     end
+
+    def to_json(json : JSON::Builder)
+      json.field "languageCode", @language_code
+      json.field "autoGenerated", @auto_generated
+      json.field "label", @label
+      json.field "body" do
+        json.array do
+          @lines.each do |line|
+            json.object do
+              if line.is_a? HeadingLine
+                json.field "type", "heading"
+              else
+                json.field "type", "regular"
+              end
+
+              json.field "startMs", line.start_ms.total_milliseconds
+              json.field "endMs", line.end_ms.total_milliseconds
+              json.field "line", line.line
+            end
+          end
+        end
+      end
+    end
+
+    def to_json
+      JSON.build do |json|
+        json.object do
+          json.field "transcript" do
+            json.object do
+              to_json(json)
+            end
+          end
+        end
+      end
+    end
   end
 end