Update x/vibc to support Agoric ibc-hooks middleware

[michaelfig]

What is the Problem Being Solved?

The current Golang x/vibc module is built only for JS contracts to use directly. It is not suited to implementing middleware to handle IBC apps that are already written in Golang.

With a little bit of work, x/vibc could also be used to intercept Golang IBC app upcalls and downcalls, and forward them to JS for handling.

Description of the Design

Implement IBC_EVENT messages destined for the JS VM that would be triggered by upcalls from Golang "Agoric ibc-hooks" middleware as well as the (currently supported) ibc-go core modules. Also implement IBC_METHOD messages destined for the JS VM that can be triggered by IBC apps that are wrapped by Golang "Agoric ibc-hooks" middleware.

Because IBC is nearly completely async-tolerant, it pairs well with async calls/replies to and from the Agoric JS VM.

Security Considerations

Middleware needs to be carefully audited. It can exert complete control over the IBC app that it wraps.

Scaling Considerations

No particularly heavy load... just the overhead of contract calls and replies when Golang passes messages over the JS VM bridge.

Test Plan

Implement vtransfer (#8624) using this support and verify it end-to-end.

Upgrade Considerations

No additional considerations beyond making middleware JS target contracts upgradable.