Custom Fields set to read-only are not usable in the filter UI

[jasonyates]

NetBox version

3.3.4

Python version

3.9

Steps to Reproduce

1. Create a custom field & set it's UI visibility to read-only
2. Visit the relevant model list and attempt to filter based on the custom field

Expected Behavior

Custom field is usable in the Filter UI even when marked read-only.

Observed Behavior

Custom field is blocked from use in a filter. Validated on public demo instance