v3 makes all schema classes based on Generic[T]
So now when models are instantiated, they will show what class they are based on
Like so:

my_model = MyModel(some_str='a')
# my_model is type MyModel[frozendict.frozendict] in the vscode IDE
# my_model.some_str is type SomeStr[str] in the vscode IDE

• keep the mixin classes for ide methods (needed to show possible properties)
• make Schema generic
• when making classes always define new so the generic can be seen
• update properties to add the generic type
• var equals class define class instead, needed for generics
• update ref props to add generic
• fix UnsetAnyTypeSchema not being used in AbstractStepMessage
• update get_item property with type
• update get_item array with type
• add __init__ method to AnyTypeSchema
• update parameter and header schema and content type definition to be [typed]
• change ref schemas to be type aliases

Out of scope future improvement. Use typing.Never in NotAnyType

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