event stream: add events for CSI volumes and plugins

[tgross]

Adds new topics to the event stream for CSI volumes and CSI plugins. We'll emit events when either is created or deleted, and when CSI volumes are claimed.

Ref: #24721

Description

Testing & Reproduction steps

In addition to the unit tests, you can use nomad operator api "/v1/event/stream?topic=CSIVolume" to poll for CSI volume events while running the host path demo.

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[gulducat]

lgtm!

[gulducat]

was ReadJob selected because the CSIPlugin contains alloc information (and whatever JobDescriptions is)?

[tgross]

Yes. Although the reason we're using it here is because the CSIPlugin.Read RPC handler uses the same permission, and they should match 😁