new action: bp_rest_api_init

[DJPaul]

I want to add a new action bp_rest_api_init hooked to WP's rest_api_init so REST API endpoint implementations requiring can hook themselves to this and avoid making some other check to see if BuddyPress exists. Much like we do for many other common actions.

[boonebgorges]

Fine to introduce it, especially for use by plugins. I think we should be cautious when deciding to use it internally (we don't need to check whether BP exists - we are BP).

[djpaul]

In 10719:

Add new bp_rest_api_init filter for future use by our REST API integration.

Fixes #7032