[identity] Query reserved usernames in FindUserIdentities RPC

Summary:
Addresses server-side of ENG-8771.
Modified FindUserIdentities RPC to also look up the reserved usernames table.

Depends on D12922, D12923

Test Plan:

• Have a few users registered to identity
• Manually added a few items to the reserved usernames table.
• Call the RPC with user IDs from both tables, and also a few non-existent IDs
• Verify that response contains usernames for both types of users

Also verified that previous methods that base on modified DatabaseClient::query_reserved_usernames_table() work the same way as before.

Reviewers: varun, will

Reviewed By: varun

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D12924