[identity] Update invalid username check on Identity service

Summary:

1. check that the username is valid (no invalid special characters, min length 1, max length 191, starts with alphanumeric character)
2. return invalid username error instead of username reserved error if username is a valid eth address

Test Plan:

1. unit tests
2. used commtest to try registering various invalid usernames, confirmed that i got an invalid username response from identity each time

Reviewers: bartek, michal, will

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D11329