Introduce RequestEviction feature for evicting pods in background

When the feature is enabled each pod with descheduler.alpha.kubernetes.io/request-evict-only
annotation will have the eviction API error examined for a specific
error code/reason and message. If matched eviction of such a pod will be interpreted
as initiation of an eviction in background.

test/e2e/test_clientconnection_test.go

@@ -31,6 +31,7 @@ func TestClientConnectionConfiguration(t *testing.T) {
 		t.Fatalf("Unable to initialize server: %v", err)
 	}
 	s.Client = clientSet
+	s.DefaultFeatureGates = initFeatureGates()
 	evictionPolicyGroupVersion, err := eutils.SupportEviction(s.Client)
 	if err != nil || len(evictionPolicyGroupVersion) == 0 {
 		t.Errorf("Error when checking support for eviction: %v", err)