Add support for extra-create-metadata when creating snapshots

[francoispqt]

What type of PR is this?
/kind feature

What this PR does / why we need it:
This PR adds metadata in the description field for Snapshots and Images created from the CreateSnapshot CSI method when the external-snapshotter is run with the --extra-create-metadata flag.

Which issue(s) this PR fixes:
Fixes #695

Does this PR introduce a user-facing change?:

Description field for Images and Snapshots will be filled with JSON encoded metadata containing the name and namespace of the VolumeSnapshot and the name of the VolumeSnapshot content.

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: francoispqt / name: Francois Parquet (460cd2a)

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[k8s-ci-robot]

Hi @francoispqt. Thanks for your PR.

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[mattcary]

/ok-to-test

Thanks for working on this! Basically looks okay to me, I'll wait for it to run through tests.

Remember to squash your commits before checking in.

[francoispqt]

Thanks for reviewing!

Sure I will squash my commits before merging.

[mattcary]

This comment is a little out of date, as PD does support labels now. To reduce confusion, could you reword this something like the following?

enocodeTags encodes requested volume or snapshot tags into a JSON string, suitable for putting into 
the PD Description field.

[francoispqt]

You're right. I have updated the comment.

I think using labels would be more convenient than using the description field (for cost monitoring for example) but even though Snapshot and PD now support labels, the constraint on label values (max length 63 char) make it unsuitable.

[mattcary]

Thanks. Let's wait for the tests to pass, squash your commits, and I'll lgtm.

[francoispqt]

I have just squashed commits

[mattcary]

/lgtm
/approve

Thanks!

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: francoispqt, mattcary

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