Updated the codebase to replace the `Signature` record with the new `DocReceiverElementCreateDto` record for better alignment with the domain model.
- Updated `EnvelopeReceiverController` to use `DocReceiverElements` instead of `Signatures` when iterating over `Receivers`.
- Replaced the `Signature` record with `DocReceiverElementCreateDto` in `CreateEnvelopeReceiverCommand`.
- Updated `ReceiverGetOrCreateCommand` to use a `DocReceiverElements` property instead of `Signatures`.
These changes ensure consistency and reflect a shift in how document-related data is represented.
