Proposal: Make get_UID Pluggable

Hi there!

I'd like to propose making `get_UID` pluggable so that users can implement their own version without affecting others.

In certain cases, it is necessary to keep track of UID changes by recording the UIDs before and after modifications over a series of anonymizations done in separate processes. Additionally, some users might want to use a specific prefix to generate their own UIDs. The current implementation doesn't allow this since `get_UID` is tightly coupled with the code, and its dictionary is volatile.

If this proposal is acceptable, I am willing to raise a PR for review. I would like to hear feedback on whether this change is agreeable.

Start by locating get_UID and the dictionary it currently uses, then trace how anonymization processes call it. Clarify the interface for user-provided UID generation, persistence across separate processes, and custom prefixes. Done should include an agreed pluggable design and coverage for those behaviors.

Written by the indexing model from the issue text.