Add support for verification of SHA256 signatures

[jayaddison]

Per the Wise webhook events documentation, a X-Signature-SHA256 HTTP header is available to verify the authenticity and integrity of webhook requests.

It'd be useful to provide a way to verify this within the library. Should the existing (deprecated) verification process be retained? At this small scale of the library's adoption I think perhaps not; better to shift to a more secure algorithm and encourage that while it's easy to make that kind of change.