Refactor `AuthController` to include `IMediator` dependency and
introduce a new `CreateTokenForEnvelopeReceiver` API endpoint
to handle envelope receiver authentication.
- Updated `using` directives to remove unused namespaces and
add required ones for new functionality.
- Added `ReceiverLogin` model to represent envelope receiver
login credentials.
- Implemented `ReadEnvelopeReceiverSecretQuery` to validate
access codes for envelope receivers.
- Cleaned up unused fields and dependencies in `AuthController`.
