Add option to set color to events in gCal

[Valentina R]

Now, all events added to gCal have the default color. Would it be possible to set a color for all events added via WP? Even just a hook with explanation of how to use it would be enough.

Requested here: https://bookingwp.com/forums/topic/google-calendar-sync-appointment-event-color/#post-174761

[Valentina R]

Duplicate of #477.