feat(ws): update Workspace and WorkspaceKind definitions

[thesuperzapper]

This PR continues from #6.

It makes the following changes to align with the updates discussed, see thesuperzapper/kubeflow-notebooks-v2-design@ee651ee for the changes.

The key changes are:

• Renaming some of the Go types because they were likely to overlap with future ones, e.g. PodVolumes became WorkspacePodVolumes.
• Add spec for labels on each spawner option, to be used as filters the UI when selecting from options.
• Add status for WorkspaceKind which lists the options and how many Workspaces are using them
• Add redirect "message" to be displayed in the UI when warning users about the update.

I have also re-ordered the types and added section headings to make the go files easier to read.

[thesuperzapper]

I cant assign @jiridanek as a reviewer because he is not a member yet, but please take a look.

[jiridanek]

look taken

/lgtm

[google-oss-prow]

@jiridanek: changing LGTM is restricted to collaborators

In response to this:

look taken

/lgtm

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[ederign]

/lgtm

[thesuperzapper]

/approve

[google-oss-prow]

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