Add ability to filter by post type at search time

[kevinfodness]

The restrict_post_types general setting actually filters which post types are eligible for being added to the index at index time, and there isn't a separate utility for filtering at search time. This PR clarifies (via comments) the intention of the restrict_post_types setting and adds a new filter for restricting post types at search time.

[dlh01]

This looks 🌳 to me as far as the code goes. To me, though, the "empty array for all" pattern seems like it requires some extra thinking. The plugin has access to the list of post types; could it pass those? As of now, the new elasticsearch_extensions_searchable_post_types filter is all-or-nothing in the sense that I couldn't use it to make a contextual decision, like "if X is present/absent, include/exclude Y." Either I supply my own complete list or I get the provider's defaults.

[kevinfodness]

That's a great point. I'll add a parameter to that function that accepts a default, which can be passed through from adapter implementation classes with whatever the provider defaults are, which can then be further modified or completely overridden by userland code.