[identity] refactor message validation

Summary:
a lot of this code will be reused later when we validate messages from the keyserver to add and remove usernames from the reserved list

this refactor makes it easier to reuse the validation code

Depends on D8231

Test Plan: tested later when the validate helper is called

Reviewers: jon, bartek

Reviewed By: bartek

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8232