vsphere agents: import OVAs via the vSphere content library instead of directly to inventory

To better support downstream users of the vmware variant of Bottlerocket, we need to make sure to test EKS-A's vSphere provisioning process as much as possible. This involves importing OVAs via vSphere content libraries through `govc library.import` instead of directly to the inventory through `govc import.ova`.

The vSphere content library apparently does additional checks against the OVA.

Start by locating the vSphere agent workflow that currently invokes `govc import.ova` and read how its OVA inputs are selected. Update the workflow to use `govc library.import` through a vSphere content library, then verify the EKS-A vSphere provisioning process and confirm the content-library checks succeed.

Written by the indexing model from the issue text.