Add admin file inspector (#12635)

Download files from *any* bucket/path as an encrypted zip file.

The key is included in the response but can be separated so zip 
and the key doesn't have to be sent on the same channel.

Requires https://github.com/minio/pkg/pull/6

cmd/storage-interface.go

@@ -74,6 +74,7 @@ type StorageAPI interface {
 	CheckFile(ctx context.Context, volume string, path string) (err error)
 	Delete(ctx context.Context, volume string, path string, recursive bool) (err error)
 	VerifyFile(ctx context.Context, volume, path string, fi FileInfo) error
+	StatInfoFile(ctx context.Context, volume, path string) (stat StatInfo, err error)
 
 	// Write all data, syncs the data to disk.
 	// Should be used for smaller payloads.