Adds get/setHasMultipleTypes to Java schema classes

[spacether]

adds get/setHasMultipleTypes to Java schema classes
This will allow us to detect the user case where multiple types are defined in one schema which is a feature of
oas 3.1.0
If generator maintainers want to, they can use this parameter when nullable is set to true and use it to set values like:

• isNull = true
• isMap = true
• getHasMultipleTypes = true

So this will support array of type use cases in specs v3.1.0

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