[prometheus-node-exporter] Allow mounting volumes in kubeRBACProxy to support BYO certs

[fullykubed]

Is your feature request related to a problem ?

In more recent versions of kube-rbac-proxy, self-signed certs are disallowed so we must bring our own certs: brancz/kube-rbac-proxy#187

Describe the solution you'd like.

Allow for arbitrary volume mounts in kubeRBACProxy block

Describe alternatives you've considered.

None

Additional context.

No response

[fullykubed]

This also makes it impossible to not use the deprecated bearerTokenFile option in the associated service monitor as it is not possible to mount a CA bundle to validate x.509 certs sent from the prometheus instances.