creating more than one file based blobstore using the same root path is not prevented

One can create multiple file based blobstores to the same root path.

Expected

We should add validation to prevent Nexus from create a second file blob store with the same root location of an existing file blob store, or any common path. For the latter, this means it should not be possible to create a file blob store whose storage is inside of another existing file blob store.

We should also run a check at startup to see if any blob stores currently violate the above constraint. If they do, we should have a health check fail, so the user is informed of the problem so they can fix it. But other than that, we should allow it, we don't want to break existing instances by enforcing this new rule on them.