Add an option to mark fields with Required attribute by default and add the Optional attribute

[Artem Perepelitsa]

I found myself adding the Required attribute to most of serialized UnityEngine.Object fields. It is a common case, as optional fields are maybe 5% of overall references in my projects. The attribute is sprinkled everywhere in the code.
I would like to have the option to have fields, eligible for the Required attribute (UnityEngine.Object, strings), be required by default. And instead, use the Optional attribute occasionally when I need it. The option would be represented as a toggle in the Preferences window.

[Tor Esa Vestergaard]

This feature is implemented in Odin Validator in the form of the "Reference Required By Default" rule.