Implement Signature Verification in JsonProductStorage

• Add a configurable SignatureVerifier to the class
• Use verifier to verify signature before adding product to storage (database)
• Update documentation to include configuration properties for signature verification
• Create unit tests
  • Valid signature case
    • Assert product is in storage after attempt
  • Invalid signature case
    • Assert product is not in storage after attempt

Use the existing FileProductStorage as a reference.