[CodeGen] Move to .NET Code Gen LROs

[annelo-msft]

Summary

Per @pakrym, the C# code generator creates Operation<T> subclasses automatically from swagger files. We'll want to understand the output and whether it can be used for the LROs in the FormRecognizer library.

Goals

• Understand the LRO output from code generator
• Determine whether existing FR LRO functionality can be implemented with only small changes to this output
• If code gen LRO code can be used for FR, implement this
• If code gen LRO code differs too much from the FR functionality, we should keep the hand-written Operation classes and close this issue.

[annelo-msft]

Do these together: #11484

[pakrym]

I think this is not valid anymore, when we moved to only generate rest clients for data-plane we stopped generating operations as well.