fix(opt-null-zoned-datetime): added serializers for zoned datetime in OptionalNullable type

[asadali214]

What

This PR adds the following changes:

• Serializers in OptionalNullable class to serialize types OptionalNullable
• Update unit tests to cover the new ZonedDateTime serializers

Why

We required serializers for ZonedDateTime when marked as OptionalNullable, this addition will fix the critical exceptions raised during the serialization of OptionalNullable<ZonedDateTime> instances

Closes #82

Type of change

Select multiple if applicable.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause a breaking change)
• Tests (adds or updates tests)
• Documentation (adds or updates documentation)
• Refactor (style improvements, performance improvements, code refactoring)
• Revert (reverts a commit)
• CI/Build (adds or updates a script, change in external dependencies)

Dependency Change

If a new dependency is being added, please ensure that it adheres to the following guideline https://github.com/apimatic/apimatic-codegen/wiki/Policy-of-adding-new-dependencies-in-the-core-libraries

Breaking change

If the PR is introducing a breaking change, please ensure that it adheres to the following guideline https://github.com/apimatic/apimatic-codegen/wiki/Guidelines-for-maintaining-core-libraries

Testing

List the steps that were taken to test the changes

Checklist

• My code follows the coding conventions
• I have performed a self-review of my own code
• I have commented my code, particularly in hard-to-understand areas
• I have added new unit tests