Typed envelopes follow-up work

[jbowes]

From the original pr (#170):

• [ ] Get a nice interface in place for the session/auth stuff (XXX included in changes)
• [x] Share api clients between frontend and daemon (#174, #175, #176)
• [ ] (Maybe) Replace envelope/primitive split with a flattened primitive only struct that will serialize and deserialize properly
• [ ] Interfaces and embedded shared structs all over the place

These were left off to reduce the size of the PR. The shared api clients point would be the most nice to have immediately.

[ianlivingstone]

👍