How to make different permissions for different subdirectories of storage bucket

On Google Cloud Storage or Amazon S3, is there a way to give users a key to only a specific subdirectory of a storage bucket, for example:

gs://images.eng.cm.com/avatars/<user-uuid>/xyz

so the user has a user id of and they can read and write only to that subdirectory of the storage bucket, and only if they have a read/write access key in hand too?

Anyone know how to create this fine-grained access control? is it possible/advised?