Support extra volume mounts in helm chart

[kate-osborn]

Is your enhancement request related to a problem? Please describe.
Add support for mounting additional volumes into the NGF and NGINX containers in the helm values file.

What would you like to be added:
The ability to define extra volume mounts for both NGF and NGINX containers in the helm values file.

Why this is needed:
From PR #1193:

We leverage internal certificates for communication between services and need to load the trusted root certificate into the containers. Mounting the certificate as a volume prevents us from having to build our own container image.