Support publishing to chainStorage-backed subscriptions

[turadg]

What is the Problem Being Solved?

On-chain vats need to publish sequential values for off-chain consumers.

Description of the Design

Create a function that accepts a chainStorage node (cf. #4558) and returns an updater/publication object (cf. https://agoric.com/documentation/guides/js-programming/notifiers.html ) such that writes end up in chain storage. Since such writes are committed only at block boundaries, It will probably be necessary to invent a container format communicating multiple results in a single chainStorage string (and possibly a way to opt out of that for publishers whose consumers are guaranteed to tolerate loss of intermediate state).

Security Considerations

We can control the publish capability, but all data written to chain storage is inherently public.

Test Plan

[turadg]

planning meetings tomorrow will save time having an estimate for this going in. @gibson042 ?

[Tartuffo]

Done.