C# - fix #19470 - emulate legacy serializer/deserializer when contextual API is used

[mgravell]

ref #19470

1. ensure that .Serializer/.Deserializer do something useful when the Marshaller<T> is created with contextual implementations
2. also use sane default implementation of PayloadAsReadOnlySequence, as not all contexts might implement correctly

This is because grpc-dotnet uses .Serializer/.Deserializer directly, see grpc/grpc-dotnet#357

[jtattermusch]

The thing is that we used to have Marshallers emulate legacy serializer/deserializer when it was constructed using the contextual API, but we removed that emulation as no-longer-needed
when Grpc.Core switched from using the ContextualSerializer and ContextualDeserializer internally.

Because the contextual API is more powerful and flexible, I think it make more sense to require all gRPC implementations to use marshaller.ContextualSerializer and marshaller.ContextualDeserializer internally. Legacy code that still uses the simple serializers/deserializers (e.g. the gRPC generated code at this point) will continue to work because of the one-way emulation that marshallers provide, but I don't see a reason to continue emulating the other way.

See this PR for context: https://github.com/grpc/grpc/pull/17167/files#diff-2058a085f39531e466d41c163feccbe5

[mgravell]

In that case, that's a topic for @JamesNK, 'cos right now: that isn't the case for grpc-dotnet

[jtattermusch]

I think the right solution is to switch grpc-dotnet to use Contextual API internally (as captured in grpc/grpc-dotnet#357).