Allow user name suffix for OAuth sources

[mxmehl]

Description

At our Gitea instance, we would like to introduce a second authentication source next to LDAP, namely Github. However, namespace-wise, we would like to give preference to our current and future LDAP users.

Can Gitea automatically add a suffix to a user name when this user registeres with a specific authentication provider, e.g. that the Github user janedoe becomes janedoe-guest or janedoe-github?

Debian's GitLab supports something like that for external users who are not Debian developers: https://signup.salsa.debian.org/register/guest/

[6543]

should be handled around here:

gitea/routers/web/user/auth.go

Line 652 in 3a6edd3

if !createAndHandleCreatedUser(ctx, base.TplName(""), nil, u, &gothUser, setting.OAuth2Client.AccountLinking != setting.OAuth2AccountLinkingDisabled) {

after #16594 got in you have the LoginSource souce to check option against