ACL policy interpolation

If the `variables.path` label in ACL policies supported templates, we could allow finer-grained control over access to secrets. This will only be especially useful once Nomad has expanded the work on [Workload Identity](https://www.nomadproject.io/docs/concepts/workload-identity)

Example:

```hcl
namespace "example" {
variables {

# full access to user-specific secrets
path "user/{{ .token.name }}" {
capabilities = ["write", "read", "destroy", "list"]
}

# list-only access for everything else
path "*" {
capabilities = ["list"]
}

}
}
```

Start by reading the linked Workload Identity documentation and the ACL policy behavior described in this issue. Determine how templates in the variables.path label should be evaluated and how that interacts with the wildcard path example. Done means user-specific paths can be matched while the broader list-only rule continues to work.

Written by the indexing model from the issue text.