Previously it only filtered on a specific rna path-based criteria.
This commit changes it to take a predicate function instead, splitting
off its previously fixed criteria into a separate function.
This also renames it to `fcurves_in_action_slot_filtered()` to better
reflect its new generalized functionality.
No functional changes intended.
Pull Request: https://projects.blender.org/blender/blender/pulls/128423
476fd1641c