Improve stores

[radiac]

Stores are mostly stubs at the moment:

• store and restore probably need to return/accept a constructor dict too
• might be nice to lock keys to make it safer. or maybe just use a dataclass.

Serialisable store class

• store() -> str - serialise to json, or to some unique ID
• restore(str) - deserialise from json or unique ID

Top-level client.store - SessionStore:

• dataclass-style. actually a dataclass?
• do we want annotations and type hinting - pydantic?
• or keep it simple for now, and just go with a dict

Needs to be extensible - user code needs to be able to add fields.