Introduce a new function to get a list of activity type labels keyed by their type names

[imath]

To migrate the Latest Activity Widget as a Block Widget, we need this type of list. Existing activity functions about getting this kind of list are not matching this need.

This function will be primary use to improve the BP REST API Components get_items endpoint to fetch this list and build the Block's Activity type multi select box setting.

[imath]

In 12987:

Introduce a new function to get a list of type labels keyed by names

bp_activity_get_types_list() returns an array of activity type labels keyed by activity type names. It satisfies a need which appeared working on a new Activity BP Block.

Fixes #8504