[sbansla]

Summary

Adds HMAC-SHA256 support to RequestValidator, for webhooks Twilio signs with a SharedKey.

Changes

• validate(...) and compute_signature(...) take an optional algorithm: HMAC_SHA1 (default) or HMAC_SHA256.
• New get_signature_algorithm(header) reads the algorithm from X-Twilio-Webhooks-Signature-Settings. It defaults to HMAC_SHA1.
• New constants for the header names and algorithms.
• An unsupported algorithm makes validate return False with a logged warning.
• Existing calls are unchanged and still use HMAC-SHA1.