[services] use username and public key from registration request

Summary:
the client will send a username and public key in the registration request that the identity service persists in dynamodb. this diff implements that behavior.

Depends on D4626

Test Plan: checked that the new data was successfully stored in dynamodb

Reviewers: tomek, karol

Reviewed By: tomek, karol

Subscribers: ashoat, adrian, atul, abosh

Differential Revision: https://phab.comm.dev/D4628