add user rename endpoint to admin api (#22789)

this is a simple endpoint that adds the ability to rename users to the
admin API.

Note: this is not in a mergeable state. It would be better if this was
handled by a PATCH/POST to the /api/v1/admin/users/{username} endpoint
and the username is modified.

---------

Co-authored-by: Jason Song <i@wolfogre.com>

services/user/user.go

@@ -27,6 +27,22 @@ import (
 	"code.gitea.io/gitea/services/packages"
 )
 
+// RenameUser renames a user
+func RenameUser(ctx context.Context, u *user_model.User, newUserName string) error {
+	ctx, committer, err := db.TxContext(ctx)
+	if err != nil {
+		return err
+	}
+	defer committer.Close()
+	if err := renameUser(ctx, u, newUserName); err != nil {
+		return err
+	}
+	if err := committer.Commit(); err != nil {
+		return err
+	}
+	return err
+}
+
 // DeleteUser completely and permanently deletes everything of a user,
 // but issues/comments/pulls will be kept and shown as someone has been deleted,
 // unless the user is younger than USER_DELETE_WITH_COMMENTS_MAX_DAYS.