Allow the JSON serializer to include public instance fields in the
output in addition to public properties, controlled by the new
JsonSerializeOptions.IncludePublicFields flag (default false to keep
existing behaviour). Field metadata is cached per type like enum and
property reflection lookups.
Closes#3920
Continues the work started for #5917 (already covered Targets, Layouts,
LayoutRenderers and Target Wrappers), extending the same pattern to the
NLog.Filters namespace so default values show up in IntelliSense and the
generated API docs.
Adds a regression test that scans all settable NLog.Filters properties
against the generated XML documentation file and fails if any of them is
missing a documented default value.