feat: Add signature functionality to ContractSigner

- Add `signature_data` field to `ContractSigner` to store base64
  encoded signature image data.  Allows for storing visual
  signatures alongside electronic ones.
- Implement `sign` method for `ContractSigner` to handle signing
  with optional signature data and comments. Improves the
  flexibility and expressiveness of the signing process.
- Add Rhai functions for signature management, including signing
  with/without data and clearing signature data.  Extends the
  Rhai scripting capabilities for contract management.
- Add comprehensive unit tests to cover the new signature
  functionality. Ensures correctness and robustness of the
  implementation.
- Update examples to demonstrate the new signature functionality.
  Provides clear usage examples for developers.

specs/models/legal/contract.v

@@ -57,6 +57,7 @@ pub mut:
 	signed_at  ourtime.OurTime // Optional in Rust, OurTime can be zero
 	comments   string        // Optional in Rust, string can be empty
 	last_reminder_mail_sent_at ourtime.OurTime // Unix timestamp of last reminder sent
+	signature_data string    // Base64 encoded signature image data (Optional in Rust)
 }
 
 // SignerStatus defines the status of a contract signer