Formalize serialization/deserialization of commands/results/objects which represent messages

[Julian]

Right now, bowtie essentially contains an NIH implementation of RPC / a serialization framework.

Essentially, a bunch of objects either call attrs.asdict or have manually authored .serializable / .from_dict / etc. methods for explaining how to serialize or load them when they are sent as JSON.

This was done more or less because I don't really have a library I like that does this -- but I'm also not super familiar with the broader attrs ecosystem, and I vaguely recall there are some libraries there (cattrs? I think that's one?) which deserve looking at at least.