[Triggers] Set a different welcome message based on widget

Currently, Triggers have no controls to choose which "chat widgets" they apply to.

This would make it very easy to set different welcome messages for different pages of the site.

To add, and I know this is a separate ask, we should be able to set it so that only one welcome message can fire in one session so as to not spam the visitor if they navigate across different pages where conditions of multiple triggers that send a message are met.