Provider support for dotnet

This adds a new namespace to the dotnet sdk "Provider" with classes to support the writing of a native dotnet provider.

The Provider class will internally deal with our grpc protocol and call into virtual methods given by the user which use only dotnet native domain models.

It is a bit sad all this is manually written, I feel like there _might_ be a good way to codegen this from the protobuf specs such that if they ever change this file could stay in sync.

Also the PropertyValue Marshal/Unmarshal duplicates a lot of the existing serialisation logic, and I think that logic could be rewritten as a layer ontop of this (that is we'd have "objects -> PropertyValues" and "PropetyValues -> grpc" rather than what we currently have which is "objects -> grpc" and now "PropertyValues -> grpc")