Helper functions that handle the PAKE login workflow.

Admittedly, there's some code from the RegisterUser workflow repeated here. Because the gRPC message is slightly different (RegistrationResponse vs LoginResponse), it's hard to handle both response types in a single function without a lot of match statements and nesting. I'll create a task to make this more DRY.

Depends on D4905

cargo build, tested later in login_user method