Change stored-directory/config to JSON

[diegonehab]

Context

At present, when we store a machine state to directory, we create a config file in protobuf format, with the corresponding machine_config that will be used to restore the machine into a new instance.

We have received requests to document the format of this config file, and even provide some tooling to inspect its contents without instantiating the machine.

Possible solutions

The easiest solution is to replace protubuf with JSON. We already have a json-util module that knows how to read and write machine_config structures, so the code changes will be minimal.

Better yet, we will remove the protubuf dependency from the machine library.
It will remain only in the soon-to-be-deprecated gRPC remote machine server, and in the client support for connecting to it, which already lives in separate library.