[#1643] added Gitea OAuth2 provider

Co-authored-by: Steve MacLeod <sjmacleoddev@gmail.com>

tools/auth/auth.go

@@ -114,6 +114,8 @@ func NewProviderByName(name string) (Provider, error) {
 		return NewLivechatProvider(), nil
 	case NameAuthentik:
 		return NewAuthentikProvider(), nil
+	case NameGitea:
+		return NewGiteaProvider(), nil
 	default:
 		return nil, errors.New("Missing provider " + name)
 	}