[identity] Modify FindUserID response to respect reserved_usernames

Summary:
Addresses ENG-5802.

Added is_reserved field to FindUserIDResponse to indicate if the username/wallet address is in the reserved_usernames list.
Followed the logic that we discussed in the Linear task.

Depends on D10052

Test Plan:

• Created a user (not-reserved) and checked the response: userID != null, is_reserved == false
• Added the same user to the reserved_usernames list and checked the response: userID != null, is_reserved == false
• Added a username to the reserved list and checked the response: userID == null, is_reserved == true
• For other cases, checked the response: userID == null, is_reserved == false

Also tested wallet addresses

Reviewers: ashoat, varun

Reviewed By: ashoat, varun

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D10053