Feature Request: Introduce flexible ap_post extensions using capabilities

### What

Use a capability based system aka, register all necessary stuff which is regitered for the ap_post post-type for others.

e.g.

```php
if ( \post_type_supports( 'activitypub-remote-post' ) {
...
}
```

### Why

Remote objects that are cached might be more nuanced that a `Note`. E.g. a Event or a Poll might want to use a dedicated post type.

The [Polls For ActivityPub plugin](https://codeberg.org/linos/wordpress-polls-for-activitypub) for example could register a dedicated post-type for remote polls, and use many of the existing features of the local poll post type but not all of them.

Also it could make this available to the reader and allow voting on incomming polls.

### How

_No response_

Start by tracing where the ap_post post type is registered and where its existing features are applied. Compare that flow with WordPress capability checks such as post_type_supports(), then define the registration and inheritance behavior needed for dedicated remote post types such as polls or events. Done means remote post types can opt into the appropriate existing features without assuming every remote object is a Note.

Written by the indexing model from the issue text.