Allow querying multiple namespaces at once

All HTTP APIs for namespaced objects (eg jobs) only allow querying against a single namespace. They should also support querying multiple namespaces at a time similar to how Policy Namespace Rules can be applied with glob pattern matching: https://learn.hashicorp.com/nomad/acls/policies#namespace-rules

*HTTP API Selection, projection, and pagination #7340* could be viewed as a generalization of this issue, but namespacing may warrant special handling given its security implications.