What is the problem you're trying to solve

Remote snapshotters like stargz, nydus, and a snapshotter we hope to soon publicly release, have to use a large amount of duplicate code from the overlay snapshotter because there is no way to access the internal MetaStore.

Describe the solution you'd like

Add the option of passing in your own MetaStore as has been done in this PR.

Additional context

We have refactored stargz-snapshotter to show the code reduction this change would allow.