client-go/test: warning handler with contextual logging

The default handler now uses contextual logging. Instead of

     warnings.go:106] warning 1

it now logs the caller of client-go and uses structured, contextual
logging

     main.go:100] "Warning" message="warning 1"

Users of client-go have the choice whether the handler that they provide uses
the traditional API (no API break!) or contextual logging.

Kubernetes-commit: 48fb886325fce4b16e4067caadb7bcd3044d460f

rest/exec_test.go

@@ -242,6 +242,9 @@ func TestConfigToExecClusterRoundtrip(t *testing.T) {
 		func(h *WarningHandler, f fuzz.Continue) {
 			*h = &fakeWarningHandler{}
 		},
+		func(h *WarningHandlerWithContext, f fuzz.Continue) {
+			*h = &fakeWarningHandlerWithContext{}
+		},
 		// Authentication does not require fuzzer
 		func(r *AuthProviderConfigPersister, f fuzz.Continue) {},
 		func(r *clientcmdapi.AuthProviderConfig, f fuzz.Continue) {
@@ -289,6 +292,7 @@ func TestConfigToExecClusterRoundtrip(t *testing.T) {
 		expected.Burst = 0
 		expected.RateLimiter = nil
 		expected.WarningHandler = nil
+		expected.WarningHandlerWithContext = nil
 		expected.Timeout = 0
 		expected.Dial = nil