Add ability to set the initial owner of a custom volume

[stgraber]

For filesystem custom volumes, we currently have root:root as the initial owner of the volume.
This makes sense for system containers and VMs and is generally the correct behavior there.

But for OCI containers where the application may be running as non-root and where there isn't an init system to put the storage in place and correct ownership, having the ability to create a volume with a pre-defined owner would be quite useful.

This would likely look like 3 additional volume keys:

• initial.uid
• initial.gid
• initial.mode