Add Webhooks & Callbacks Support

[sufyankhanrao]

Description
Introduce foundational support for Webhooks and Callbacks in the Python core library.

What to Add

• Framework-agnostic request snapshot.
• Generic, configurable HMAC verifier.
• JSON Pointer–based parsing templates.
• Unit tests and minimal documentation.

What to Test

• HMAC verifier with different configurations (headers, order, delimiter, encoder, hash algorithm).
• JSON Pointer resolution for headers, query params, and body.
• Immutability and case-insensitive header access in request snapshot.

Goal
Enable SDKs to process incoming webhook events securely.