Make "value" attributes filterable

People might want to filter the value attribute so that they can store the ID instead of the slug for taxonomy_checkbox fields, for example

No file, test, or entry point is named. Start by locating how taxonomy_checkbox fields handle their value attribute and how filtering is currently applied; clarify the intended filtering API and ID-versus-slug behavior before implementation, then verify that value attributes can be filtered for the stated use case.

Written by the indexing model from the issue text.