Implement `getAuthMessage` functionality from kavach module

[unniznd]

Overview

Implement the getAuthMessage function in the Python SDK to get the authentication message for signing

Function Signature

const getAuthMessage = async (address: string): Promise<AuthMessage>

Excepted Response Format

{"message":<auth_message_if_success>, "error":<error_if_any>}

• error is None if successful

Reference

• encryption-sdk/src/methods/getAuthMessage/index.ts at main · lighthouse-web3/encryption-sdk
• encryption-sdk/src/methods/getAuthMessage/test/getAuthMessage.test.ts at main · lighthouse-web3/encryption-sdk

[unniznd]

PR has been made #34