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.

heromodels/examples/legal_contract_example.rs

@@ -159,5 +159,66 @@ fn main() {
         }
     }
 
+    // Demonstrate signature functionality
+    println!("\n--- Signature Functionality Demo ---");
+
+    // Simulate signing with signature data
+    if let Some(signer_to_sign) = contract.signers.get_mut(1) {
+        println!("\nBefore signing:");
+        println!(
+            "  Signer: {} ({})",
+            signer_to_sign.name, signer_to_sign.email
+        );
+        println!("  Status: {:?}", signer_to_sign.status);
+        println!("  Signed at: {:?}", signer_to_sign.signed_at);
+        println!("  Signature data: {:?}", signer_to_sign.signature_data);
+
+        // Example base64 signature data (1x1 transparent PNG)
+        let signature_data = "data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAAEAAAABCAYAAAAfFcSJAAAADUlEQVR42mNkYPhfDwAChwGA60e6kgAAAABJRU5ErkJggg==".to_string();
+
+        // Sign the contract with signature data
+        signer_to_sign.sign(
+            Some(signature_data.clone()),
+            Some("I agree to all terms and conditions.".to_string()),
+        );
+
+        println!("\nAfter signing:");
+        println!("  Status: {:?}", signer_to_sign.status);
+        println!("  Signed at: {:?}", signer_to_sign.signed_at);
+        println!("  Comments: {:?}", signer_to_sign.comments);
+        println!(
+            "  Signature data length: {} characters",
+            signer_to_sign
+                .signature_data
+                .as_ref()
+                .map_or(0, |s| s.len())
+        );
+        println!(
+            "  Signature data preview: {}...",
+            signer_to_sign
+                .signature_data
+                .as_ref()
+                .map_or("None".to_string(), |s| s
+                    .chars()
+                    .take(50)
+                    .collect::<String>())
+        );
+    }
+
+    // Demonstrate signing without signature data
+    if let Some(first_signer) = contract.signers.get_mut(0) {
+        println!("\nSigning without signature data:");
+        println!("  Signer: {}", first_signer.name);
+
+        first_signer.sign(
+            None,
+            Some("Signed electronically without visual signature.".to_string()),
+        );
+
+        println!("  Status after signing: {:?}", first_signer.status);
+        println!("  Signature data: {:?}", first_signer.signature_data);
+        println!("  Comments: {:?}", first_signer.comments);
+    }
+
     println!("\nLegal Contract Model demonstration complete.");
 }