Add RawInput to workaround double-serialization issue

[jviau]

This PR is to address a conflicting need in Durable Functions extension. We want to use TaskHubClient from within LocalGrpcListener so that we leverage all the logic there, especially the distributed tracing work. However, our input is already serialized and TaskHubClient would double serialize it. To work around this an "internal" API called RawInput has been added. When this type is used as the input for TaskHubClient, we will directly use RawInput.Value instead of serializing the input.

RawInput is "internal" in the sense that it is public, but in an .Internal namespace, with an [Obsolete] attribute, and xmldoc advising customers to use the type at their own risk. This is a pattern used in a few .NET libraries (efcore, webjobs, functions worker, etc) to address the internal-yet-public functionality.

[davidmrdavid]

This makes sense to me. Tagging @cgillum for a final approval

[cgillum]

LGTM - I assume part of the reason why we're marking this [Obsolete] is because it's not really a fully fleshed-out feature (can't use RawValue in any other contexts currently)?

[jviau]

I assume part of the reason why we're marking this [Obsolete] is because it's not really a fully fleshed-out feature (can't use RawValue in any other contexts currently)?

Essentially - it is used to raise a flag to customers that while this type is public, we don't want them to use it. So, every usage of it comes with a pseudo "I agree" via suppressing the warning.