Algo for hashing: add SHA256withRSA to sha256

[Richard2012]

The vendor code and the documentation use sha256 as algoritm for hashing. This is fine if you operate both server and client for the webhooks.

A large player in the webhook business, Paypal, uses SHA256withRSA on their servers. The rest of the world must customize, for hours and hours. I suggest you include an option to hash with SHA256withRSA. The grateful community would send you an ocean of beautiful postards.