mod_auth_oauth_external: Allow different username in PLAIN vs final JID Mastodon for example having email addresses usernames in login, but a different username in the service itself. Thanks to @tcit@social.tcit.fr for the pointer to a usable validation endpoint for Mastodon, allowing this to be tested.

module:depends("cloud_notify_encrypted");
module:depends("cloud_notify_priority_tag");
module:depends("cloud_notify_filters");