Implement a CLI sync manager tool

[thomcc]

This is mostly to 'prove out' the sync manager API, and help ensure nothing is missing / finish up the non-clients parts that are missing from #1379.

It will also (hopefully) prove to be helpful for debugging future sync issues similar to our other CLI tooling, or, aspirationally, about:sync, although it will certainly not be as fully featured as about:sync (at least not initially).

This can mostly happen in parallel with the clients engine work (#1380), but needs #1379 first. (As with the clients engine work, this will probably be a rolling PR, and so will have some inherent concurrency to it).

This is part of the sync manager work (#1217).

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