Make sure all component main classes have a `block_init()` method

[imath]

Working on new blocks from the BP Blocks plugin I've noticed, this was lacking in previous versions of BuddyPress. When not method is available into the BP_Component extended class, it's not possible to filter the BP Blocks to eventually add new ones.

This ticket will init the Widget Block merge process.

[imath]

In 12994:

Make sure all BP components init their BP Blocks

Even if some components don't have native BP Blocks, it makes it easier for developers to add their custom blocks or override existing BP Blocks or disable BP Blocks. Working on the BP Blocks feature as a plugin made us realize this.

This commit also improves the BP_Block constructor inline documentation, adds a new method to the Core component to register REST controllers and moves the BP_REST_Components_Endpoint one from the Members component to the Core component.

Fixes #8511