See if upstream wants to make overlayfs snapshotter embeddable

[elpdt852]

So at a high-level, nix-snapshotter is basically overlayfs snapshotter but with additional support for nix layers.

Currently there's a lot of code duplication between pkg/nix/nix.go and the overlayfs snapshotter from containerd: https://github.com/containerd/containerd/blob/main/snapshots/overlay/overlay.go

This is because we use private members like o.ms:

	ctx, t, err := o.ms.TransactionContext(ctx, false)

to handle the metadata layer's bolt transactions.

If we look around the remote snapshotter ecosystem, you'll see this is duplicated the same way:
https://github.com/containerd/stargz-snapshotter/blob/main/snapshot/snapshot.go

We should consider upstreaming a refactor to make it possible to embed it, thus deleting a lot of code from pkg/nix/nix.go.

[elpdt852]

Summarizing our offline discussion:

• We have two types of dependency on containerd. An IPC-level one (based on GRPC protocol-level compatibility), and a library-level one (consuming containerd golang packages)
• We should create an upstream GitHub issue as that will take a long time
• We should fork containerd to make the changes to overlay snapshotter so that it's embedabble
• We can point to this fork to greatly reduce code in pkg/nix/nix.go without breaking our IPC-level dependency (i.e. we can build nix-snapshotter against the fork but still run as a snapshotter for a release version of containerd)

[elpdt852]

@rbpdt This has been merged, congrats! We should unfork containerd in our go.mod to close this issue.

containerd/containerd#8945