Add system Nexus signal-with-start workflow API

[tconley1428]

Summary

• generate system Nexus workflow APIs from temporary WIT during the SDK build
• expose generated operations from the Workflow partial class without checking generated C# into source
• handle system Nexus envelopes with binary protobuf outer payloads and generated nested payload traversal for codecs
• add a focused workflow test for signal-with-start from workflow with a payload codec

Testing

• dotnet build src/Temporalio/Temporalio.csproj -f netstandard2.0 -clp:ErrorsOnly -v:q
• dotnet test tests/Temporalio.Tests/Temporalio.Tests.csproj --filter FullyQualifiedName~ExecuteWorkflowAsync_SignalWithStartFromWorkflow_Succeeds -clp:ErrorsOnly -v:n

[jmaeagle99]

I'm slightly concerned about the fragility of this. It uses regex on the C# output from the nex-gen call, the message structure from the proto descriptors, and does some kind of fuzzy-ish join on the two. A couple of things:

• Can we have nex-gen emit the operation to message mapping as some kind of structured data and read that instead?
• Can this project be refactored a bit to be testable? It would be good to have this generator be testable and to add tests for the generated payload visitor.

[tconley1428]

We talked about this. I think we're going to replace it with a more generic payload visitor soon as needed for external storage and that should replace this brittle generator.

[jmaeagle99]

I would ask that the ExpressionUtil forward and marking public methods / types as experimental are done before merging. Adding GeneratedCode attribute would be nice, but can be deferred.