Make extension snapshotter interface safer to use

[yihuang]

Summary

CosmWasm/wasmd#816 (comment)
When implementing wasm extension snapshotter, we found that "a plugin can break another one without knowing" with current interface.

Problem Definition

The Restore api expects the extension to return the last parsed item if it's not an extension payload message, because that could be the extension boundary, extension implementer may not be aware of such contract, better to enforce it with the API design.

Proposal

Create SnapshotExtension and RestoreExtension methods for ExtensionSnapshotter, which will handle payload stream specifically.

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