Add attribute flag building and parsing (#1562)

This adds a few command sketches but that's not the main point of the PR
and filling those out will come in a different PR. This adds a new flag
type that can combine inputs to a single target slice, retaining the
ability to distinguish where it came from but also keep ordering across
different flag names. It also implements a function that does the
heuristic parsing with fallback.

internal/servers/controller/handlers/targets/target_service_test.go

@@ -2599,12 +2599,12 @@ func TestAuthorizeSession(t *testing.T) {
 				}
 				return &plgpb.ListHostsResponse{Hosts: []*plgpb.ListHostsResponseHost{
 					{
-						SetIds: setIds,
+						SetIds:      setIds,
 						ExternalId:  "test",
 						IpAddresses: []string{"10.0.0.1", "192.168.0.1"},
 					},
 					{
-						SetIds: setIds,
+						SetIds:      setIds,
 						ExternalId:  "test2",
 						IpAddresses: []string{"10.1.1.1", "192.168.1.1"},
 					},