This changeset adds a new core job `CoreJobCSIVolumePublicationGC` to
the leader's loop for scheduling core job evals. Right now this is an
empty method body without even a config file stanza. Later changesets
will implement the logic of volume publication GC.

This commit introduces support for staging volumes when a plugin
implements the STAGE_UNSTAGE_VOLUME capability.

See the following for further reference material:
 https://github.com/container-storage-interface/spec/blob/4731db0e0bc53238b93850f43ab05d9355df0fd9/spec.md#nodestagevolume

This commit adds helpers that create and validate the staging directory
for a given volume. It is currently missing usage options as the
interfaces are not yet in place for those.

The staging directory is only required when a volume has the
STAGE_UNSTAGE Volume capability and has to live within the plugin root
as the plugin needs to be able to create mounts inside it from within
the container.

This package introduces some basic abstractions around mount utilties
for various platforms. Initially it only supports linux, but the plan is
to expand this as CSI expands across to other platforms.

* structs: CSIPlugin indexes jobs acting as plugins and node updates

* schema: csi_plugins table for CSIPlugin

* nomad: csi_endpoint use vol.Denormalize, plugin requests

* nomad: csi_volume_endpoint: rename to csi_endpoint

* agent: add CSI plugin endpoints

* state_store_test: use generated ids to avoid t.Parallel conflicts

* contributing: add note about registering new RPC structs

* command: agent http register plugin lists

* api: CSI plugin queries, ControllerHealthy -> ControllersHealthy

* state_store: copy on write for volumes and plugins

* structs: copy on write for volumes and plugins

* state_store: CSIVolumeByID returns an unhealthy volume, denormalize

* nomad: csi_endpoint use CSIVolumeDenormalizePlugins

* structs: remove struct errors for missing objects

* nomad: csi_endpoint return nil for missing objects, not errors

* api: return meta from Register to avoid EOF error

* state_store: CSIVolumeDenormalize keep allocs in their own maps

* state_store: CSIVolumeDeregister error on missing volume

* state_store: CSIVolumeRegister set indexes

* nomad: csi_endpoint use CSIVolumeDenormalizePlugins tests

Fix comment typo
Co-Authored-By: Tim Gross <tim@0x74696d.com>

This Commit adds the VolumeMounter interface, and the VolumeManager
type.

The volume manager will handle the lifecycle of all volumes for a given
CSIPlugin on the node.

This commit extracts the fingerprint logic from the csi instance manager
into its own struct.

This change is to facilitate reusing the csimanager to also manage the
node-local CSI functionality, as it is the easiest place for us to
guaruntee health checking and to provide additional visibility into the
running operations through the fingerprinter mechanism and goroutine.

Implements a fake version of NodeStageVolume as a dependency of
implementing the client.NodeStageVolume request

This commit introduces a nomad model for interacting with CSI
VolumeCapabilities as a pre-requisite for implementing NodeStageVolume
and NodeMountVolume correctly.

These fields have a few special characteristics that I've tried to model
here - specificially, we make a basic attempt to avoid printing data
that should be redacted during debug logs (additional mount flags), and
also attempt to make debuggability of other integer fields easier by
implementing the fmt.Stringer and fmt.GoStringer interfaces as
necessary.

We do not currnetly implement a CSI Protobuf -> Nomad implementation
transformation as this is currently not needed by any used RPCs.