Generate an `__eq__` method for case insensitive enums

**Is your feature request related to a problem? Please describe.**
Using `==` when testing equality of strings to case-insensitive enum types like
`if my_model.change_type == StackModels.DeploymentStacksWhatIfPropertyChangeType.ARRAY:`
tests it case sensitively. Same result if the operands are flipped. See case insensitive enums in https://pypi.org/project/azure-mgmt-resource-deploymentstacks/ @ 1.0.0

**Describe the solution you'd like**
The python code generator will generate an `__eq__` method on case-insensitive enums that does a case-insensitive equality check if the other value is a `str`.

**Describe alternatives you've considered**
Have to write a helper function and remember to use it. Can manually call `lower()` on both operands but python has no `?.` operator if the value could be null. Having overridden operator will keep things simpler.

**Additional context**
It's not consistent with other SDK language code gen, such as .NET, which implements these as structs with an overridden equality method that ignores case.

Inizia individuando la logica del generatore di codice Python per gli enum senza distinzione tra maiuscole e minuscole e confronta il suo output con gli enum senza distinzione tra maiuscole e minuscole di azure-mgmt-resource-deploymentstacks 1.0.0. Aggiungi la copertura per l’uguaglianza con stringhe in entrambi gli ordini degli operandi, includendo maiuscole e minuscole differenti, e assicurati che l’enum generato implementi il comportamento richiesto.

Scritto dal modello di indicizzazione a partire dal testo della issue.