[FormRecognizer] Consider adding parameterless protected constructors to Operation types

[kinelski]

Regarding our mocking guidelines: https://azure.github.io/azure-sdk/dotnet_introduction.html#dotnet-mocking

DO provide protected parameterless constructor for mocking.
DO provide factory or builder for constructing model graphs returned from virtual service methods.

The last point suggests that we create a factory for models because "model types shouldn’t have public constructors". This doesn't apply to Operation types, since they have a public constructor to access an existing LRO.

We're wondering whether we should include a parameterless protected constructor in Operations as well. It seems Storage is doing so (in Storage Blobs, at least), but Key Vault isn't, so I feel like our guidelines are not very specific on this matter. It would be great to update our guidelines to be more explicit about that, and decide on a preferred approach, if possible.

There's currently an open discussion about that in the .NET language channel.