Improve performance of ActorState by generating JsonContext for it #138
Description
This can be used by a storage provider that can more efficiently serialize it to JSON, such as the streamstone one for the snapshot.
The tricky part is that if we want to emit a single JsonSerializerContext-derived class for the whole app (ideal since the storage provider must deal potentially with multiple grains), we'd need to emit it at the app level. But if we do so, we'd need to take a dependency on the STJ generator and generators cannot depend on each other: this means our emitted JSON context won't be seen by the built-in STJ generator and it won't work.
So this might require quite a bit more work than it might seem initially.
An alternative is to emit a context per actor/state, but then you'd have an explosion of configuration since each actor/grain would need to use a specific storage provider that's been configured with a specific JsonOptions :(
