Support VolumeContentSource.VolumeSource for CreateVolume calls

[saikat-royc]

PD supports creation of disks from source disks.
https://cloud.google.com/sdk/gcloud/reference/compute/disks/create#--source-disk

PD CSI driver should support this capability

CSI spec for CreateVolume request: https://github.com/container-storage-interface/spec/blob/master/csi.proto#L332

[saikat-royc]

/cc @mattcary

[msau42]

#161

[fejta-bot]

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/lifecycle stale

[fejta-bot]

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Mark the issue as fresh with /remove-lifecycle rotten.
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/lifecycle rotten

[halradaideh]

any updates here ?

[mattcary]

/lifecycle frozen

This is in our queue but no specific date yet. (PRs welcome :-)

[mattcary]

/assign @amacaskill

Alexis is working on this now, yay!

[k8s-ci-robot]

@mattcary: GitHub didn't allow me to assign the following users: amacaskill.

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In response to this:

/assign @amacaskill

Alexis is working on this now, yay!

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[amacaskill]

/assign @amacaskill

[mattcary]

dup of #161, which is now closed