Replace baroque orchestrator serialize/deserialize code

[jackjansen]

Followup to #117

The current code is a weird set of things that manually decodes and encodes parameters as JSON.

I'm pretty sure that we can have a simple interface where (almost) every call Foo has a FooRequest and FooReply type than are classes that can be jsonified in one step.

There's one or two "funny" calls that are basically unsolicited replies, for example when a new user joins a session.

But I think if we have an orchestrator API that has an interface definition for the outgoing API and an interface definition for the callbacks we're in a situation where it because rather easy to replace the whole orchestrator by something completely different.

[jackjansen]

Other things that need cleanup:

• converting SessionConfig.ProtocolType by using names like dash internally (same as the string known by the orchestrator).
• Numeric UserRepresentationType values are ugly.

[jackjansen]

For the numeric values https://code-maze.com/csharp-serialize-enum-to-string/ has some hints.