[FormRecognizer] Update DocumentAnalysisModelFactory to autogenerate models

[kinelski]

Currently both old (v2) and new (v3) Form Recognizer APIs have their model factory generation disabled in the autorest configuration file.

Ideally, we should be able to:

• Reenable model factory generation for the new API.
• Add the CodeGenModel("FormRecognizerModelFactory") attribute to the DocumentAnalysisModelFactory class. This will make all generated models to be put into the new factory.
• Remove as many duplicates as we can from the manually written side of the DocumentAnalysisModelFactory class. We want to maximize the amount of generated code.

There's one issue with the approach above: the old FormRecognizerModelFactory already has a CodeGenType("FormRecognizerModelFactory") attribute. If we add the attribute to the new factory, autorest fails due to the duplicated keys. If we remove the attribute from the old factory, ApiCompat fails.

Removing this internal attribute from the old factory is not considered a breaking change, so we need to suppress this error somehow.