Add kernel bootstrapping parameters

[rekmarks]

Requires #203 #29

We know that we will need to to test and demo the kernel in various ways. Therefore, it would be useful to tell the kernel how to bootstrap arbitrary object graphs (backed up by user code running in vats) on or after initialization. It should be possible to do this with a simple configuration file, probably in the form of a JSON blob. A similar feature exists for Agoric's Swingset, which they have used extensively.

[rekmarks]

@FUDCo I think we kind of have this already, it's just not parameterized?

[rekmarks]

So, at the moment, we have a cluster bootstrapping method Kernel.launchSubcluster(), which launches a pre-configured cluster of 3 vats. This is great for testing, but this is the thing that we want parameterized. In addition, this method should not exist as such, but probably be decomposed into multiple lower-level method calls that originate from the extension (or @ocap/nodejs, as the case may be).

The "bootstrap cluster" should be configurable, probably via some kind of config file. This file can be served locally, just as we are serving vat user code locally in order to launch the subcluster today. By changing the served configuration file, a different bootstrap cluster should be launched.

In addition, #353 and related work have added a call to rawStorage.clear() in the kernel constructor. This ensures that a fresh bootstrap cluster will be installed and run every time the kernel is restarted. This should ideally also be configurable, perhaps via a flag to the build system (or a property in the config file).

Possibly, this issue needs to be decomposed into multiple sub-issues.