Add support for alternative domains for Invidious cookies (#5647)

* Add support for alternative domains for Invidious cookies

* check if @@secure is already true before changing it's value

* Add alternative_domains config option example

config/config.example.yml

@@ -151,6 +151,26 @@ db:
 ##
 domain:
 
+##
+## List of alternative domains where the invidious instance is being served.
+## This needs to be set in order to be able to login and update user preferences
+## when using a domain that is not the same as the `domain` configuration,
+## like a .`onion` address, `.i2p` address, `.b32.i2p` address, etc.
+##
+## It will detect the alternative domain trough the `X-Forwarded-Host` header.
+## https://developer.mozilla.org/en-US/docs/Web/HTTP/Reference/Headers/X-Forwarded-Host
+##
+## Accepted values: a list of fully qualified domain names (FQDN)
+## Default: <none>
+##
+## Example:
+## alternative_domains:
+##   - invidious.example.com
+##   - inv.example.com
+##   - videos.example.com
+##
+alternative_domains:
+
 ##
 ## Tell Invidious that it is behind a proxy that provides only
 ## HTTPS, so all links must use the https:// scheme. This