Support synchronising extension's global state

[sandy081]

Support synchronising extension's global state

[bwateratmsft]

For Docker extension, we'd need to be able to sync on a per-key basis. There are some areas where sync would be nice to have (nothing imperative, right now), but there are some where we cannot sync (e.g. machine-specific information that we want to cache forever because it's expensive to compute but never changes).

[legomushroom]

This could be useful for Live Share to sync up the chat history (currently stored locally for single machine only) cc @jramsay @fubaduba

[sandy081]

Proposal:

Provide a contribution point for the extension to register memento keys to sync

"contributes": {
		"settingsSync.memento": {
			"global": [
				"key1"
			]
		}
}

[bwateratmsft]

I like that plan. We do have some keys in the Docker extension that are determined at runtime (thus couldn't be in package.json), but I don't think any of them need synchronization.