Support for child namespaces

### Proposal

Vault supports having a root namespace with childs under it. That's very useful for multi tenancy and helps teams organize their stuff in a hierarchical way. It would be very nice if we could match namespaces 1:1 between nomad and vault. This way each namespace would have it own vault and would be under the exact same structure, making it much easier to track.

### Use-cases

Complex systems tend to need isolated environments. This happens either because you need pre prod and prod envs or because you need to isolate different clients. It's possible to create child namespaces in vault than it makes sense to have the same feature in nomad. This way we can keep naming consistent between the two products and properly separate the environments.

### Attempted Solutions

We can use a mixed solution of child namespaces in vault matching independent namespaces in nomad, but it needs some workarounds to get all the naming right and makes much harder to automate that task. In the end it's much more complicated than it looks like and doesn't gives you the hierarchy you have in vault namespaces.

Start by reviewing Nomad's existing namespace model and its Vault integration, then compare them with Vault's hierarchical namespace behavior. Define how parent and child namespaces should be represented and matched, including isolation and naming, and validate the design against the stated multi-tenancy and environment use cases.

Written by the indexing model from the issue text.