fix: correct default values for different data types

[isha689]

Fixes #663

Set the default values for different data types correctly. Previously, for string, bool, int, DateTime the default value had been set as Values:: NONE which is string. Each dataType should have its correct default value. DateTime is set as null because we cannot set an object as a default function/method parameter.

Related PR: https://github.com/twilio/twilio-oai-generator/pull/423

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