Adds isDateTime to CodegenModel

[spacether]

This PR:

Adds isDateTme to CodegenModel
adds isDateTime getter and setter to IJsonSchemaValidationProperties
Adds a test to ensure that isDateTime has the correct values on on implementing java classes
is a feature break out for #7651
Note: this is potentially a breaking change because if one is generating a model for a date-time schema, the current master branch sets isString in that CodegenModel to true. After this PR, isString will be False and isDateTime will be true, which is consistent with how date schemas are handled in these classes:

CodegenProperty
CodegenParameter
CodegenResponse
Setting isString and isDateTime for date schemas should be consistent in these 4 classes.

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