Split RuntimeManagerMessage type, unify with IceCap messages

[dominic-mulligan-arm]

Requested feature

The RuntimeManagerMessage type from veracruz-utils should be split into distinct RuntimeManagerRequest and RuntimeManagerResponse types. Moreover, the special-purpose analogue of this type for IceCap should be unified with the Nitro/Linux types.

[gbryant-arm]

There are already the RuntimeManagerRequest and RuntimeManagerResponse structures which are the messages that are (de)serialized by protobuf and used to communicate between the client and the runtime manager. Should we pick another name for the messages between the server and the runtime manager? Something that reflects that the communication is more control oriented?