Transparent network proxy installs via install-config

1. Proposed title of this feature request

Transparent network proxy installs via install-config

2. What is the nature and description of the request?

Provide an install-config recommendation for folks installing clusters that require day-0 access to a proxy defined at the network level, but which requires an X.509 CA not found in OpenShift's default trust store.

3. Why does the customer need this? (List the business requirements here)

Currently folks who want to install clusters with transparent, network-level proxies need to wrangle with openshift-install create manifests ..., unless their cluster can get far enough in for them to make day-2 changes via the cluster's Kubernetes API. Folks periodically trip over this, and end up complaining to the installer about surprising additionalTrustBundle handling. Providing more convenient configuration with more discoverable documentation will reduce customer pain tripping over the existing UX wrinkle, and will reduce installer noise responding to customer complaints.

4. List any affected packages or components.

Just the installer.