Expose MapUserFn for NewCustomServer

[cdanyl]

Using NewCustomServer how can use MapUserFn? https://github.com/go-pkgz/auth/blob/4e32f30d2628c11f9104b1467c8d82cf968228dd/v2/provider/custom_server.go#L49 to be able to do something like https://github.com/go-pkgz/auth/blob/78537e6f812df08d95ae8a5f9c049dfb59247b2e/v2/provider/providers.go#L33

My problem using a customServer, i am blocked with an error provider is not allowed.

A custom provider with name provider doesn't create a user.id equal to provider_1234567890 https://github.com/go-pkgz/auth/blob/cea049c06aa5be34e1e23bc4bf927b8680ca068b/middleware/auth.go#L159

func newOauth2Provider(
	goauth2 *goauth2.Server,
	oauth2Config *config.Oauth2,
	oauth2loginPageHandler http.HandlerFunc,
	oauth2Logger *OAuth2Logger,
) *provider.CustomServer {
	serverOption := provider.CustomServerOpt{
		URL:              oauth2Config.Server,
		L:                oauth2Logger,
		LoginPageHandler: oauth2loginPageHandler,
	}

	prov := provider.NewCustomServer(goauth2, serverOption)

	return prov
}

server.AddCustomProvider("provider", client, oauth2Provider.HandlerOpt)

Maybe i am doing something wrong, but i think is good idea to expose MapUserFn

Maybe related : https://github.com/umputun/remark42/issues/1660#issuecomment-1685330652