In #24528 we added monitoring to the CLI for dynamic host volume creation. But
when the volume's namespace is set by the volume specification instead of the
`-namespace` flag, the API client doesn't have the right namespace and gets a
404 when setting up the monitoring. The specification always overrides the
`-namespace` flag, so use that when available for all subsequent API calls.
Ref: https://github.com/hashicorp/nomad/pull/24479
Most Nomad upsert RPCs accept a single object with the notable exception of
CSI. But in CSI we don't actually expose this to users except through the Go
API. It deeply complicates how we present errors to users, especially once
Sentinel policy enforcement enters the mix.
Refactor the `HostVolume.Create` and `HostVolume.Register` RPCs to take a single
volume instead of a slice of volumes.
Add a stub function for Enterprise policy enforcement. This requires splitting
out placement from the `createVolume` function so that we can ensure we've
completed placement before trying to enforce policy.
Ref: https://github.com/hashicorp/nomad/pull/24479
