Support EdDSA jwts in the Share auth lib.

[ChrisPenner]

Overview

• Set up a new EDDSA secret on staging
• Set up new EDDSA secret on prod
• Set the env var in the deployment hcls for share-api
• Set the env var in the deployment hcls for cloud-api
• Need to update the lib in cloud-api
• Then actually do a key rotation to a new EdDSA key
• Deploy the new cloud-api FIRST
• Deploy share-api next

We want to be able to support public/private key crypto so that BYOC instances can validate tokens which come from Share.

This tweaks the auth library to allow EdDSA as a JWT algorithm,
and while I'm at it, adds key id and algo verification and key rotation.

Implementation notes

• Tweaks the defaultJWTSettings helper, it now accepts a key definition, alongside a 'legacy key' and list of keys which we've rotated.
• Also paves the way towards per-user keys if that's something we end up supporting (it can be useful for long-lived clients and/or API tokens)

Test coverage

• Test key rotation and new keys.

[ChrisPenner]

crypton is where cryptonite moved to since HVR is no longer maintaining it.