HostVolumePlugin interface and two implementations

[gulducat]

This is functional end-to-end from CLI to Client, provided you place the example-host-volume plugin script in /opt/nomad/hostvolumeplugins/ on the client node, or use the "mkdir" internal plugin.

I have not tried this on MacOS or Windows, only my Linux laptop. Perhaps @pkazmierczak can test it on Mac for us? :slightly_smiling_face:

There are many TODOs around, and no unit tests are present. Testing what's here is up next on my todo list, provided the overall shape of the interface here looks good to y'all.

Ref: #24479

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The _test-plugin.sh script was convenient for me sprucing up the example plugin, and gave me two ideas for future:

• a similar nomad CLI subcommand might be neat to include, to ease plugin development for users
• the dir that will contain these plugins could skip files that start with _ (or . or something) for library files
  • this could encourage big complicated bash monsters, though, so maybe not

[tgross]

@gulducat if you want to push testing into it's own PR that feels reasonable for a large PR on something so "side-effect-y" but we should probably add the package to the list in ci/test-core.json to make the linter happy?

[gulducat]

I've started writing tests, but I'm also happy to open another PR for it, so we can all collectively use this purportedly-functional thing in the meantime :stuck_out_tongue_winking_eye: