Support for deferring critical headers to the caller for validation.

[joesiltberg]

Here's an attempt to solve #288.

I've used functional options (https://github.com/tmrts/go-patterns/blob/master/idiom/functional-options.md) in order to add options to the Verify*-functions without breaking backwards compatibility.

[CLAassistant]

All committers have signed the CLA.

[joesiltberg]

Note that backwards compatibility is still technically broken since the signature is changed, so this shouldn't be back ported to 2.x

[joesiltberg]

@mbyczkowski @csstaub should I somehow request a review or mark the pull request as ready for review?

Thanks!

[csstaub]

Hi @joesiltberg, I'm working on moving dev on the main branch to a new repo in a separate org (will retain v1/2 in this repo, and v3 will be tagged in a new repo that's in a shared org outside of Square). Hang on while I get that set up.