Filter objects with select/multiselect custom fields by choice PK

[glennmatthews]

Closes #5009

What's Changed

Added the option to filter objects with select/multi-select custom fields based on the UUID of the defined custom field choice(s), for example /api/dcim/locations/?cf_multiselect=1ea9237c-3ba7-4985-ba7e-6fd9e9bff813 as an alternative to /api/dcim/locations/?cf_multiselect=some-choice-value.

• Added CustomFieldSelectFilter class with custom get_filter_predicate() implementation
• Changed CustomFieldMultiSelectFilter to be a subclass of CustomFieldSelectFilter
• Removed CustomFieldMultiValueSelectFilter as it's confusing, totally unused, and not part of the apps namespace.
• Changed extended filter for multi-select custom fields from __n to __nic to match the fact that the base filter here is icontains, not exact. (I'm second-guessing this now, but it "feels" like a bug fix - thoughts??) reverted this out of caution
• Add unit tests covering the new functionality.

Screenshots

TODO

• Explanation of Change(s)
• Added change log fragment(s) (for more information see the documentation)
• Attached Screenshots, Payload Example
• Unit, Integration Tests
• Documentation Updates (when adding/changing features)
• n/a Example App Updates (when adding/changing features)
• Outline Remaining Work, Constraints from Design