Add options for secure webhooks

[dmitrijivanchenko]

Description

• Add option "signing secret" for #create and #update webhooks methods

Checklist

• [ ] Tests (if applicable)
• [ ] Documentation (if applicable)
• [ ] Changelog stub (or use conventional commit messages)

[dmitrijivanchenko]

I think, we can add an example of signature validation function to the docs. Ideally, it may be a helper/utility function of the lib (not API) next to generating other signatures. What do you think?

Yeah, good idea, I'll add one