[identity] db and siwe verification changes to handle login requests from keyserver

Summary:
DB changes: If a given user does not already exist, we change the way we construct the update expression to avoid a validation exception from DynamoDB

SIWE changes: The signature from keyserver is hex encoded, so we decode it to a Vec<u8>

Depends on D6943

Test Plan:
called the update_users_table method from the loginUserPake and loginUserWallet functions in the keyserver,
both succeeded

Reviewers: jon, bartek, tomek

Reviewed By: jon

Subscribers: ashoat, atul

Differential Revision: https://phab.comm.dev/D6945