Server CSI Client + Routing

[endocrimes]

As part of configuring CSI Volumes, Nomad Servers will have to make requests via a CSI Controller Plugin running on a Remote Client.

Because only a subset of Nomad Clients will be running an eligible plugin, we need the following:

• An API Shim to make programming against the CSI API easier
• That will look up an eligible client based on the plugin name, type, and datacenter
• Will then perform the required ClientRPC to that node (via server forwarding if necessary).

[tgross]

Closed by #7107, #7036, #7117

[github-actions]

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