Add Webhooks & Callbacks Contracts

[sufyankhanrao]

Description
Define the core contracts for Webhooks and Callbacks.

What to Add

• Verifier interface contract for implementing custom signature verifiers.
• Framework-agnostic request model capturing headers, method, path, body, and raw bytes.
• Verification result model to standardize success/failure states with error details.
• Base types to support event parsing and extensibility for future implementations.

Goal
Provide stable, language-agnostic contracts to ensure consistent implementation across SDK core libraries.