Add oauth login hint param

auth/provider/oauth/oauth.go

@@ -41,6 +41,10 @@ func (o *oauth) Endpoint(opts ...provider.EndpointOption) string {
 		params.Add("state", options.State)
 	}
 
+	if len(options.LoginHint) > 0 {
+		params.Add("login_hint", options.LoginHint)
+	}
+
 	if clientID := o.opts.ClientID; len(clientID) > 0 {
 		params.Add("client_id", clientID)
 	}

auth/provider/provider.go

@@ -28,7 +28,10 @@ type Grant struct {
 }
 
 type EndpointOptions struct {
+	// State is a code to verify the req
 	State string
+	// LoginHint prefils the user id on oauth clients
+	LoginHint string
 }
 
 type EndpointOption func(*EndpointOptions)
@@ -38,3 +41,9 @@ func WithState(c string) EndpointOption {
 		o.State = c
 	}
 }
+
+func WithLoginHint(hint string) EndpointOption {
+	return func(o *EndpointOptions) {
+		o.LoginHint = hint
+	}
+}