Introduce `SignCommand` and `SignCommandHandler` in the `EnvelopeGenerator.Application.Signature.Commands` namespace.
- `SignCommand` is a CQRS command that encapsulates the envelope receiver's information (`EnvelopeReceiverDto`) and optional PSPDFKit annotation data.
- Added computed properties for email template type (`TemplateType`) and receiver's email address (`EmailAddress`), with validation for null receivers.
- `SignCommandHandler` implements the `IRequestHandler<SignCommand>` interface, with a placeholder `Handle` method delegating processing to pipeline behaviors.
- Added necessary `using` directives for MediatR, DTOs, extensions, and constants.
