Adds hasRequired to all Java schema classes

[spacether]

Adds hasRequired to all Java schema classes
@wing328

This is needed if one is generating classes for object schemas and one only wants to generate the class if required exists.
One could almost do this with #requiredVars but that doesn't work because using #requiredVars mean we move into the context of iterating over each var when we just need to know if there are required vars.

hasRequired is tested in the below models:

• CodegenModel
• CodegenProperty
• CodegenParameter
• CodegenResponse

These tests show that hasRequired (and requiredVars) sometime loose the schema required information as shown in
#8906

This PR came out of work in #8928

This will be used in:
#8325

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[spacether]

The circleci DartDioPetstoreClientLibFakeTests is unrelated to this change