Implement abstract Block facet feature class

[oscarssanchez]

Description of the Change

This PR implements two abstract classes for the Facets feature. It tries to gather as many commonalities as possible from the facets so in the future, if we need to add subsequent Facet types, we can use the abstract classes as a guide to reduce developing times.

Closes #3472

How to test the Change

Changelog Entry

Changed - Moved methods to abstract facet class

Credits

Props @oscarssanchez, @felipeelia

Checklist:

• I agree to follow this project's Code of Conduct.
• I have updated the documentation accordingly.
• I have added tests to cover my change.
• All new and existing tests pass.

[oscarssanchez]

As a next step, we could think about get_facet_item_value_html() and make a decision if it makes sense to take out the "is_selected" values and merge them into one array, or if we just want to handle them separately.

Both have implications specially in the original Facet feature block. We'll need to deprecate those parameters accordingly.

[felipeelia]

@oscarssanchez I've pushed a commit changing a few things but it seems we still need to change the check_facets_rest_permission call in other Block.php files. Also, do you mind calling me on slack to explain more about that $is_selected parameter? I remember we talked about it but now that I looked into it, it seems unnecessary.