Use correct serializer options when serializing and deserializing

[alexgav]

We weren't always using correct serializer options when serializing and deserializing in cohosting. E.g. deserializing to VS LSP types should use VS LSP converters, and deserializing to Roslyn LSP types should use Roslyn LSP converters. Otherwise, things mostly work, but you can end up with base versions of the types instead of VSInternal subtypes.

I found this issue when I realized that VSInternalClientCapabilities wasn't getting deserialized quite right - top level was VSInternal version, but all fields (and their fields) were not properly deserialized to their VSInternal versions.

Also adding a couple of common helper methods to make serialization/deserializion code easier to keep consistent.

### Summary of the changes

-Common helpers for serialization and deserialization
-Make sure correct converters are used for both VL LSP and Roslyn LSP types

[davidwengier]

Probably didn't strictly need to be two separate sets of converters, and conversion functions, but not worth worrying about. I genuinely hope to remove half of this code in the next few weeks :)