Fixes #2669.

On deserialization, missing values for message types
are replaced with a "default" message.

This changes how we approach JSON formatting in general - instead of looking  at the field a value came from, we just look at the type of the value. It's possible this *could* be slightly inefficient, but if we start caring about JSON performance deeply, we'll probably want to rewrite all of this anyway. It's definitely simpler this way.

When we support dynamic messages, we'll need to modify JsonFormatter to handle enum values, as they won't come be "real" .NET enums at that point. It shouldn't be hard to do though.

We'll see what I've missed when CI fails...

- We do still generate the message types, as otherwise reflection breaks, even though it doesn't actually use those types.
- JSON handling hasn't been implemented yet

... and some implementation changes to go with them.

Generated code in next commit.