Add support for XChaCha-Poly1305 (XC20P)

[Hélder Pestana]

Hey, I was wondering if you're willing to accept a merge request to allow using XC20P as a content-encryption algorithm. It is still not part of the latest JWE RFC specs, but according to this - https://www.rfc-editor.org/rfc/rfc8439.html - and this - https://datatracker.ietf.org/doc/html/draft-irtf-cfrg-xchacha-01 - it is already something that is becoming widely accepted.

Also, the golang x/crypto already has an implementation for the XChaCha algorithm, so the changes shouldn't be too impactful.

Thanks