Adds isDate to CodegenModel

[spacether]

This PR:

• Adds isDate to CodegenModel
• adds isDate getter and setter to IJsonSchemaValidationProperties
• Adds a test to ensure that isDate has the correct values on on implementing java classes
• is a feature break out for [REQ] Implement needed common isX properties in java Schema classes in openapi core #7651

Note: this is potentially a breaking change because if one is generating a model for a date schema, the current master branch sets isString in that CodegenModel to true. After this PR, isString will be False and isDate will be true, which is consistent with how date schemas are handled in these classes:

• CodegenProperty
• CodegenParameter
• CodegenResponse

Setting isString and isDate for date schemas should be consistent in these 4 classes.

Checklist

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