interop: Introduce env var for xDS dualstack support and add xDS interop config

internal/envconfig/xds.go

@@ -56,9 +56,7 @@ var (
 
 	// XDSDualstackEndpointsEnabled is true if gRPC should read the
 	// "additional addresses" in the xDS endpoint resource.
-	// TODO: https://github.com/grpc/grpc-go/issues/7866 - Control this using
-	// an env variable when all LB policies handle endpoints.
-	XDSDualstackEndpointsEnabled = false
+	XDSDualstackEndpointsEnabled = boolFromEnv("GRPC_EXPERIMENTAL_XDS_DUALSTACK_ENDPOINTS", false)
 
 	// XDSSystemRootCertsEnabled is true when xDS enabled gRPC clients can use
 	// the system's default root certificates for TLS certificate validation.

test/kokoro/psm-dualstack.cfg

@@ -0,0 +1,17 @@
+# Config file for internal CI
+
+# Location of the continuous shell script in repository.
+build_file: "grpc-go/test/kokoro/psm-interop-test-go.sh"
+timeout_mins: 360
+
+action {
+  define_artifacts {
+    regex: "artifacts/**/*sponge_log.xml"
+    regex: "artifacts/**/*.log"
+    strip_prefix: "artifacts"
+  }
+}
+env_vars {
+  key: "PSM_TEST_SUITE"
+  value: "dualstack"
+}