Add a json_prepare_type-{post_type} filter on the /posts endpoint

This is intended mainly as the stimulus for a discussion. We currently
have the following filters on posts:

- `json_prepare_post`: Runs on anything requested via /posts
- `json_prepare_{post_type}`: Runs on anything requested via and endpoint
  extending `WP_JSON_CustomPostType`

Right now, if you don't make a custom endpoint and request a CPT, you have
to filter on `json_prepare_post` and conditionally check for the target
post type.

Having the filters exist per-endpoint is fine (although I feel like I've
seen a situation where another endpoint hit json_prepare_post, via a
call to `WP_JSON_Posts`'s `prepare_post`, I cannot find it atm). However,
I wanted to raise the question of whether we should be able to filter
against CPT objects retrieved via /posts?type= separately from
`json_prepare_posts`, and, if so, whether that filter should also be
invoked/invokable from a custom endpoint.

This implementation is set up to add a new filter method:

- `json_prepare_type-{post_type}`

This method processes any request passed through the `/posts` endpoint.

My question:

Should this be applied to objects retrieved through custom endpoints?
*i.e.*, should `json_prepare_type-{post_type}` be called from within
`WP_JSON_CustomPostType`'s `prepare_post`? If so, this would mean that

- `json_prepare_post` would be called for anything on `/posts`
- `json_prepare_{post_type}` would be called for anything on a
  custom endpoint
- `json_prepare_type-{post_type}` would be called for any custom
  type returned from the system, regardless of through which endpoint
  it was requested.

I personally like this approach, because it lets you pick your level of
specificity with a filter: whether it should handle all resources of a
type, or whether it should handle all resources from a custom endpoint
(or the default one). It makes things more consistent with the filter
`json_prepare_attachment`, as well, since that's then effectively a
`json_prepare_{post_type}` call, just as is `json_prepare_post` is for
that endpoint.

Thoughts?