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.

This module discards typing notifications sent to a bare host JID,
preventing error replies to be sent. These errors are harmless but can
be annoying sometimes if your client shows them prominently.