Add select_terms field

[theenoahmason]

Add support for the ability to set a term from a dropdown.

When set on post types, there should be an argument to actually attach the taxonomy, and retrieve the terms for that post when populating the value.

When setting on other terms, ensure that the filtering and sorting logic does not affect them. Something to look out for is whether the query comes before the actual get_terms call for the page. WP Backstage adds actions to the first get terms and get terms (count) queries to be able to target the list table calls, without affecting the others. If the filters are called first, this will mess things up, so make sure to check that this isn't the case.