[NEW] Add internal API to handle room announcements

[gdelavald]

Adds some form of modularity to the Room Announcements so they can be changed easily by other packages and even other Apps.

Room Announcement example:

import { RocketChatAnnouncement } from 'meteor/rocketchat:lib';

const newAnnouncement = new RocketChatAnnouncement({
   room: _rid, // REQUIRED: The room ID which the announcement is set
   message: "New Room Announcement", // REQUIRED: the message displayed, currently does not support markdown
   callback: "myCustomCallback", // OPTIONAL: a callback implemented by your app/feature that triggers when the user clicks the Announcement
   style: "" // OPTIONAL: currently supports the default (when style is null/undefined), warning and error
})
newAnnouncement.save(); //updates the room with the new announcement

Result:

With style: "error"

With style: "warning"