The `nomad plugin status :plugin_id` command lists allocations that implement
the plugin being queried. This list is filtered by the `-namespace` flag as
usual. Cluster admins will likely deploy plugins to a single namespace, but for
convenience they may want to have the wildcard namespace set in their command
environment.
Add support for handling the wildcard namespace to the CSI plugin RPC handler.
Fixes: https://github.com/hashicorp/nomad/issues/20537
