acls: reduce permissions of client agent virtual policy (#23304)

Nomad client agents run as privileged processes and require access to much of
the cluster state, secrets, etc. to operate. But we can improve upon this by
tightening up the virtual policy that use for RPC requests authenticated by the
node secret ID. This changeset removes the `node:read`, `plugin:read`, and
`plugin:list` policy, as well as namespace operations. In return, we add a
`AllowClientOp` check to the RPCs the client uses that would otherwise need
those policies.

Where possible, the update RPCs have also been changed to match on node ID so
that a client can only make the RPC that impacts itself. In future work, we may
be able to downscope further by adding node pool filtering to `AllowClientOp`.

Ref: https://github.com/hashicorp/nomad-enterprise/issues/1528
Ref: https://github.com/hashicorp/nomad-enterprise/pull/1529
Ref: https://hashicorp.atlassian.net/browse/NET-9925

.semgrep/rpc_endpoint.yml

@@ -40,7 +40,8 @@ rules:
 
       # Pattern used by endpoints that are used only for client-to-server.
       # Authorization can be done after forwarding, but must check the
-      # AllowClientOp policy
+      # AllowClientOp policy; the AllowClientOp condition is left open so that
+      # additional ACL checks can be made (ex. to scope to a given node/pool).
       - pattern-not-inside: |
           aclObj, err := $A.$B.AuthenticateClientOnly($A.ctx, args)
           ...
@@ -48,7 +49,7 @@ rules:
             return err
           }
           ...
-          if !aclObj.AllowClientOp() {
+          if !aclObj.AllowClientOp()
             return structs.ErrPermissionDenied
           }
           ...