[BIQ-763] add Option type with implicit get/set, remove JsonUndefined

[perf2711]

add Option type with implicit get/set, remove JsonUndefined

Issue : BIQ-763

Description

A supplement to #256 which simplifies getters and setters in the BaselineOverride class.

Note: this change MAY result in breaking changes:

var override = new BaselineOverride {...};

// browser is now of type Option<string?>, which IS a breaking change
var testName = override.TestName;
// this WILL fail
testName.toLower();
// this WILL NOT fail, as implicit conversion is made
var equal = testName == "abc";

// this will also work fine
string? testName = override.TestName;
override.TestName = "abc";
new BaselineOverride 
{
  TestName = "abc",
  Browser = null,
}

Types of Changes

What types of changes does your code introduce? Keep the ones that apply:

• Refactor/improvements

Testing strategy

• run unit tests
• run integration tests