OAuth2 plugin: Option to fetch keys from JWKS URL

[Teo Klestrup Röijezon]

We're trying to set up RabbitMQ to authenticate services using JWT service account tokens (provisioned by an internal service). Currently RabbitMQ's rabbitmq_auth_backend_oauth2 plugin only supports loading keys from the RabbitMQ config file. We'd like to have the provisioning service "own" the key store, to allow us to better rotate these keys without having to maintain the key store of every service that verifies them.

I'm hoping to get a patch up for discussion in the coming days, but bear with me if my Erlang is a bit rusty..

[Steve Mitchell]

It looks like it is supported now: https://github.com/rabbitmq/rabbitmq-server/tree/master/deps/rabbitmq_auth_backend_oauth2. 

"The key set can also be retrieved dynamically from a URL serving a JWK Set. In that case, the configuration will look like this:"

```

[ {rabbitmq_auth_backend_oauth2, [ {resource_server_id, <<"my_rabbit_server">>}, {key_config, [ {jwks_url, <<"https://my-jwt-issuer/jwks.json">>} ]} ]}, ].

```