Fix issues with nullable types

[martincostello]

Tracking issue for supporting nullable value/reference types correctly.

Refer to the details in the below issues to resolve this issue:

• [Bug]: Required and nullable properties are not nullable in the OpenAPI JSON #2877
• SupportNonNullableReferenceTypes not working for nested record types #2758
• "nullable" is missing in response schema on primitive nullable type #2731
• Schema Do not take in account Nullable Record inside record  #2698
• Nullability gets ignored for return types and dictionary values #2680
• Extend support for nullable reference types in responses #2650
• Nullable enum does not populate nullable #2633
• 'Required' remains unpopulated for nullable record properties (dotnet7) #2623
• Dictionary values are always nullable. #2587
• Nullability is not respected for form parameters #2564
• Regression when updating from v5 to v6 regarding nullable types #2503
• [regression] nullabillity problems wih dictionaries since 6.3.0 #2436
• [BUG] Missing OpenAPI spec nullable in classes with refs #2225
• Nullability with using [FromForm] on a class doesn't work #2157
• Nullable enum does not generate nullable property when using StringEnumConverter #2110
• Incorrect nullable annotation for array of nullable items #2059
• Configure non-nullable types as required #2036
• SupportNonNullableReferenceTypes not working for query parameters #2019