worker: Support client_credentials grant type in client

This will allow us to use the service accounts which work against
identity.api.openshift.com. These are much easier to manage, especially
with the new multi-tenancy, as there's a single page to create/expire
them across an account.

They also have the added benefit of not expiring automatically when
they're not used like offline tokens, and immediate expiration when
desired.

test/cases/api.sh

@@ -1294,6 +1294,7 @@ EOF
 cat <<EOF | sudo tee "/etc/osbuild-worker/osbuild-worker.toml"
 [authentication]
 oauth_url = http://localhost:8081/token
+client_id = "rhsm-api"
 offline_token = "/etc/osbuild-worker/token"
 EOF