Add ability to set po_token and visitordata ID (#4789)

This PR adds two new config option, to pass a PO token (config 'po_token') and
a visitor ID (config 'visitor_data') to Youtube. These two strings are required
to play videos using the WEB client.

Warning: These strings gives much more identifiable information to Google!

If the po_token setting is filled in, then the WEB client is used. If not, the
Android client is used. TvHtml5ScreenEmbed will still be used as a fallback.

Script for generating po_token and visitor_data:
https://github.com/iv-org/youtube-trusted-session-generator

Helps with issue 4734

src/invidious/yt_backend/youtube_api.cr

@@ -320,6 +320,10 @@ module YoutubeAPI
       client_context["client"]["platform"] = platform
     end
 
+    if CONFIG.visitor_data.is_a?(String)
+      client_context["client"]["visitorData"] = CONFIG.visitor_data.as(String)
+    end
+
     return client_context
   end
 
@@ -477,6 +481,9 @@ module YoutubeAPI
       "playbackContext" => {
         "contentPlaybackContext" => playback_ctx,
       },
+      "serviceIntegrityDimensions" => {
+        "poToken" => CONFIG.po_token,
+      },
     }
 
     # Append the additional parameters if those were provided
@@ -609,6 +616,10 @@ module YoutubeAPI
       headers["User-Agent"] = user_agent
     end
 
+    if CONFIG.visitor_data.is_a?(String)
+      headers["X-Goog-Visitor-Id"] = CONFIG.visitor_data.as(String)
+    end
+
     # Logging
     LOGGER.debug("YoutubeAPI: Using endpoint: \"#{endpoint}\"")
     LOGGER.trace("YoutubeAPI: ClientConfig: #{client_config}")