[MM-34076] Add Plugin API User custom status (#17435)

* add missing client for custom user status

* add custom user status plugin api

* update based on feedback review

* add GetCustomStatus

* update interfaces

app/status.go

@@ -406,6 +406,10 @@ func (a *App) UpdateDNDStatusOfUsers() {
 }
 
 func (a *App) SetCustomStatus(userID string, cs *model.CustomStatus) *model.AppError {
+	if cs == nil || (cs.Emoji == "" && cs.Text == "") {
+		return model.NewAppError("SetCustomStatus", "api.custom_status.set_custom_statuses.update.app_error", nil, "", http.StatusBadRequest)
+	}
+
 	user, err := a.GetUser(userID)
 	if err != nil {
 		return err
@@ -439,6 +443,15 @@ func (a *App) RemoveCustomStatus(userID string) *model.AppError {
 	return nil
 }
 
+func (a *App) GetCustomStatus(userID string) (*model.CustomStatus, *model.AppError) {
+	user, err := a.GetUser(userID)
+	if err != nil {
+		return &model.CustomStatus{}, err
+	}
+
+	return user.GetCustomStatus(), nil
+}
+
 func (a *App) addRecentCustomStatus(userID string, status *model.CustomStatus) *model.AppError {
 	var newRCS model.RecentCustomStatuses