chore(storage): generate GAPIC under internal subdir (#4018)

Add a gRPC Go GAPIC for the Storage API under an internal subdirectory of the submodule to ensure no one can import it. 

This GAPIC is being added for proof of concept development and should not be used in production or by any other client library.

Generated with the following:

```bash
$ cd google-cloud-go
$ export GOOGLE_CLOUD_GO=$(pwd)
$ cd internal/gapicgen
$ go run cloud.google.com/go/internal/gapicgen/cmd/genbot \
  -local \
  -regen-only \
  -only-gapics \
  -gapic=cloud.google.com/go/storage/internal/apiv1 \
  -gocloud-dir=$GOOGLE_CLOUD_GO
```

internal/gapicgen/generator/config.go

@@ -1222,4 +1222,12 @@ var microgenGapicConfigs = []*microgenConfig{
 		// GA after 2021/06/10
 		releaseLevel: "beta",
 	},
+	{
+		inputDirectoryPath:    "google/storage/v1",
+		pkg:                   "storage",
+		importPath:            "cloud.google.com/go/storage/internal/apiv1",
+		gRPCServiceConfigPath: "google/storage/v1/storage_grpc_service_config.json",
+		apiServiceConfigPath:  "google/storage/v1/storage_v1.yaml",
+		releaseLevel:          "alpha",
+	},
 }

internal/gapicgen/generator/config_test.go

@@ -33,8 +33,9 @@ var apivExceptions = map[string]bool{
 }
 
 var packagePathExceptions = map[string]bool{
-	"cloud.google.com/go/longrunning/autogen":   true,
-	"cloud.google.com/go/firestore/apiv1/admin": true,
+	"cloud.google.com/go/longrunning/autogen":    true,
+	"cloud.google.com/go/firestore/apiv1/admin":  true,
+	"cloud.google.com/go/storage/internal/apiv1": true,
 }
 
 // TestMicrogenConfigs validates config entries.