Add https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/extension/oidcauthextension

It applies to using Keycloak for Auth on the receiver side.

The oauth2client extension is already in the collector for the exporter side.

It's not a customer request (yet).

Being able to use Keycloak makes it easy to setup and secure a central collector with exporters in different clusters/nodes.