Support multiple keys for jwt authentication

[humaidk2]

Currently ejabberd errors out if you provide multiple keys for jwt auth.

It would be nice, to have support to verify any key based on jwt auth best practice, as this would allow key rotation,

ejabberd/src/ejabberd_options.erl

Line 470 in c291c20

econf:fail({bad_jwt_key_set, Path});

It should be able to read multiple keys, select the correct key and verify the token based on kid.

The new way of doing things is to accept a url and query it to find list of keys and to validate based on those keys

Sources:

https://auth0.com/docs/secure/tokens/json-web-tokens/json-web-key-sets
https://stytch.com/blog/understanding-jwks/

Edit: Added sources