[blob-service] DB: Add holder management methods

Summary:
Part of ENG-4269.

Added methods to manage holder assignments - design described in-depth in the notion doc attached to the parent linear issue

Depends on D8450.

Test Plan:
Tested together with the previous and the following diff

Created a custom testing sandbox to play with these:

1. Checkout this stack (this + next diff)
2. Apply this patch and copy the tests.rs file contents attached there to the services/blob/src/database/client/tests.rs.
3. cargo run

Reviewers: michal, varun, jon, patryk

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8451