Public key auth for JWTs

[greenape]

Per https://blog.miguelgrinberg.com/post/json-web-tokens-with-public-key-signatures via @maxalbert, we'd be better off using a public key solution for the JWTs as opposed to the current shared secrets.

Should -

• allow supplying public/private key to auth manager
• Maybe support generating a key pair inside the auth manager
• Provide a mechanism to get the public key from the auth manager
• Make use of the aud key in the JWT to ensure that tokens are valid only on specific servers