This change adds support for attaching network block device (NBD) to
both LinuxContainer and LinuxPod.
For LinuxContainer, whether to use the underlying
`VZNetworkBlockDeviceStorageDeviceAttachment` is determined by the URL
of the container Mount source.
For LinuxPod, adds additional API to support pod-level volumes that can
be mounted into multiple containers. The PodVolume type provides enum to
support multiple types of volume source. LinuxContainer can reference
the pod level volume using the `Mount.sharedMount()` constructor with
the name referencing the name of the pod volume. This will allow the NBD
to be attached to the pod at the VM level and then bind-mounted into the
container.
For integration tests, added a lightweight NBD server implementation in
swift that speaks the NBD protocol to ensure there is sufficient
coverage for the changes introduced.
Apple Virtualization NBD support documentation:
https://developer.apple.com/documentation/virtualization/vznetworkblockdevicestoragedeviceattachment
Michael Crosby