csharp: allow verification of webhook signature without the need of a WebHeaderCollection

[esskar]

Provide an overload to the Webhook.Verify function to except an Func<string, string> function that acts as an function provider.

Motivation

Creating an extra WebHeaderCollection everytime a webhook signature needs to get verified is unnecessary overhead for environments where the WebHeaderCollection is not part of underlying http stack; The object needs to get created just for the verification.

Solution

Allowing to pass a header provider function to avoid creating a WebHeaderCollection in asp.net core environments

See also #1616