Mastodon OAuth implementation has no handling for the email field.

[chizutan5]

According to my code reading skills, the mastodon oauth implementation on Goth has no handing for the email field, this creates what I believe is a bug on Gitea where you cannot create a gitea account with a mastodon oauth session because it will ask for the email, however you can link a mastodon authententication session to an existing gitea account which already will have an email attached.
I have documented this issue at go-gitea/gitea#21710

Pleroma (or akkoma/newroma/rebased) instances and possibly mastodon too (I am not able to test), which are otherwise compatible with mastodon auth clients, will not provide an email to the client and as such cause this problem.
We should take the user's fediverse userid (user@instance.tld) and apply this to the email field, in the same way as pronouns.page handles its own custom mastodon login, or optionally allow the user to enter their own email address in.

I would offer to pull request but I don't know the first thing about go, I was only barely able to get a custom php implementation with mediawiki working with much outside help.