Introduced a new pipeline behavior class `EnvelopeReceiverResolutionBehavior` to resolve and validate the `EnvelopeReceiver` during the signing process.
- Added necessary `using` directives for dependencies such as `AutoMapper`, `MediatR`, and `IRepository`.
- Implemented the `Handle` method to query the database for `EnvelopeReceiver` if not provided in the `SignCommand` request.
- Throws a `NotFoundException` if the `EnvelopeReceiver` is not found.
- Maps the retrieved entity to a DTO and sets it in the request.
- Ensures the behavior executes before other signing process behaviors.
