test(node-api): Introduce E2E tests for existing beacon endpoints

[nidhi-singh02]

Summary by CodeRabbit

• New Features

  • Enhanced the Validator interface with new methods for retrieving validator attributes such as public key and effective balance.
  • Introduced multiple new getter methods for improved access to epoch-related data.

• Improvements

  • Improved mock implementation to align with the updated Validator interface, enhancing testing capabilities.
  • Streamlined mock structure for easier interaction and testing.