[blob] Store indexable tag in DDB for holders
Summary:
Part of ENG-9367.
It's described in the issue why this is needed.
Test Plan:
- Unit tests
- Added some holders with prefixes. Verified that prefix is stored in DDB
Reviewers: kamil, varun, will
Reviewed By: kamil, varun
Subscribers: ashoat, tomek
Differential Revision: https://phab.comm.dev/D13615