Add volume.relocate command

[danielkucera]

Is your feature request related to a problem? Please describe.
I would like to use govc for migrating CNS volumes from one datastore to another as we need to discontinue the old datastore.

Describe the solution you'd like
Add a command volume.relocate for which one needs to specify volumeId or Name and the target datastore and govc will call the RelocateVoume function and migrate the volume

Describe alternatives you've considered
I tried to write my own implementation here but I think more people can benefit if this is included in govc.
I also tried to use cloud-native-storage-self-service-manager but it's not easy to setup, it's not very mature and it is complicated so I don't trust it very much.

Additional context
This is related to #2051

[github-actions]

Howdy 🖐   danielkucera ! Thank you for your interest in this project. We value your feedback and will respond soon.

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[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Mark as fresh by adding the comment /remove-lifecycle stale.

[danielkucera]

/remove-lifecycle stale

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Mark as fresh by adding the comment /remove-lifecycle stale.

[danielkucera]

/remove-lifecycle stale

[github-actions]

This issue is stale because it has been open for 90 days with no
activity. It will automatically close after 30 more days of
inactivity. Mark as fresh by adding the comment /remove-lifecycle stale.

[danielkucera]

/remove-lifecycle stale
this is fun