Add structs and fields to support the Nomad Pools Governance Enterprise
feature of controlling node pool access via namespaces.
Nomad Enterprise allows users to specify a default node pool to be used
by jobs that don't specify one. In order to accomplish this, it's
necessary to distinguish between a job that explicitly uses the
`default` node pool and one that did not specify any.
If the `default` node pool is set during job canonicalization it's
impossible to do this, so this commit allows a job to have an empty node
pool value during registration but sets to `default` at the admission
controller mutator.
In order to guarantee state consistency the state store validates that
the job node pool is set and exists before inserting it.
Add a new job admission hook for node pools that enforces the pool exists on
registration. Also provide the skeleton function we need for Enterprise
enforcement functions we'll implement later.
