feat: add option to allow non-local users to change usernames (#8714)

Add a new config option for OAuth2 authentication sources: allow users to change their username.

In the case where OAuth2 is more like a social OAuth2 login there's no need to not allow users to change their username. The information how the user is linked to the authentication source is stored in different fields.

Resolves forgejo/forgejo#687

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8714
Reviewed-by: 0ko <0ko@noreply.codeberg.org>
Reviewed-by: Earl Warren <earl-warren@noreply.codeberg.org>
Co-authored-by: Gusted <postmaster@gusted.xyz>
Co-committed-by: Gusted <postmaster@gusted.xyz>

models/user/fixtures/login_source.yml

@@ -0,0 +1,8 @@
+-
+  id: 1001
+  type: 6 # OAuth2
+  name: OAuth2 authentication source
+  is_active: 1
+  cfg: '{"Provider":"invalid","ClientID":"invalid","ClientSecret":"invalid","AllowUsernameChange":true}'
+  created_unix: 1753740851
+  updated_unix: 1753740851

models/user/fixtures/user.yml

@@ -11,6 +11,7 @@
   must_change_password: false
   login_source: 1001
   login_name: 123
+  login_type: 6
   type: 5
   salt: ZogKvWdyEx
   max_repo_creation: -1