To log in a user with PAKE, the Identity service will need to fetch the user's PAKE registration data from DynamoDB. This diff introduces a method to do that. The user ID is the table's primary key, so we use that to call get_item. Then we deserialize the bytes stored in DDB to get a ServerRegistration object. If the client is unable to get a ServerRegistration object, it logs the error and returns None.
Depends on D3920