[identity] db method to check if username is taken

Summary:
pretty straightforward... this diff adds a DB method that checks if
a username is taken. the implementation in D7365 wasn't exactly correct...
we can't call get_item with a secondary index, so we have to call query instead

Test Plan:
called the method from main and confirmed that it returned the
correct bool

Reviewers: jon, bartek

Reviewed By: jon

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7531