chore(internal/serviceconfig): add service config path for showcase (#3609)

Override service config for showcase since it does not live in
googleapis.

Fixes #3610

Author: JoeWang1127

internal/librarian/rust/codec_test.go

@@ -653,13 +653,14 @@ func TestToSidekickConfig(t *testing.T) {
 				Roots: []string{"showcase"},
 			},
 			channel: &config.Channel{
-				Path: "google/showcase/v1beta1",
+				Path: "schema/google/showcase/v1beta1",
 			},
 			want: &sidekickconfig.Config{
 				General: sidekickconfig.GeneralConfig{
 					Language:            "rust",
 					SpecificationFormat: "protobuf",
-					SpecificationSource: "google/showcase/v1beta1",
+					SpecificationSource: "schema/google/showcase/v1beta1",
+					ServiceConfig:       "schema/google/showcase/v1beta1/showcase_v1beta1.yaml",
 				},
 				Source: map[string]string{
 					"showcase-root": "",

internal/serviceconfig/api.go

@@ -445,5 +445,5 @@ var APIs = []API{
 	{Path: "google/storagetransfer/v1"},
 	{Path: "google/type"},
 	{Path: "grafeas/v1"},
-	{Path: "schema/google/showcase/v1beta1"},
+	{Path: "schema/google/showcase/v1beta1", ServiceConfig: "schema/google/showcase/v1beta1/showcase_v1beta1.yaml"},
 }