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/config.cr

@@ -132,6 +132,11 @@ class Config
   # Use Innertube's transcripts API instead of timedtext for closed captions
   property use_innertube_for_captions : Bool = false
 
+  # visitor data ID for Google session
+  property visitor_data : String? = nil
+  # poToken for passing bot attestation
+  property po_token : String? = nil
+
   # Saved cookies in "name1=value1; name2=value2..." format
   @[YAML::Field(converter: Preferences::StringToCookies)]
   property cookies : HTTP::Cookies = HTTP::Cookies.new