@searchable directive integration with filter search form

[mayteio]

We can probably switch out the GET_LIST query from listPosts to searchPosts and build the correct filter structure for graphql if there's a conventional query parameter for searching.

q as a search parameter is used in the List#filter react-admin docs.

Additionally, we could provide an <RaModelSearchForm /> or something that abstracts this away so it just works without the user having to worry about using the right query param in the first place.