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.