Add new Omnichannel event handlers

[Renato Becker]

Guys, we need to be able to handle the following Omnichannel events through the Apps-Engine:

• On Create / Upate Visitor
• OnSaveGuestInfo -> Here
• OnSaveRoomInfo -> Here
• OnTransferRoom-> Here

Also, we need to be able to request and send the chat transcript through the Apps-Engine:

• Here is the method to set custom fields

I'm bringing it up because we have planned to implement new Apps over the next quarter and those events/methods are essential to achieve this goal.

• Another missing feature: Set/Update Visitor Custom Fields;

[Cuong Huu Nguyen]

it is the same problem with #199

[Douglas Gubert]

Event onTransferRoom has been added to version 1.17 - available now in Rocket.Chat 3.6.0