Support new @JsonProperty.isRequired for overridable definition of "required-ness"

[cowtowncoder]

Describe your Issue

Now that @JsonProperty has "nullable" isRequired property (see FasterXML/jackson-annotations#284), JacksonAnnotationIntrospector needs to support it.

Logic is such that:

1. if isRequired has value other than OptBoolean.DEFAULT, it is used as THE choice of required-ness
2. otherwise if required = true, property is required
3. otherwise (required = false), either
   • Property is NOT required (Jackson 2.x)
   • Property MAY OR MAY NOT BE required wrt annotation (higher-level defaults used)

This logic is specifically meant to allow finer-grain overridability with working defaults: before this change, setting of @JsonProperty.required -- and often, specifically, its default (false) setting was used.

[cowtowncoder]

^^^ @k163377 I'll get this done today or tomorrow.

[cowtowncoder]

Ok: implemented now, so @JsonProperty(isRequired = OptBoolean.TRUE) (etc) work.

Noteworthy on defaulting:

• In both 2.19 and 3.0, isRequired has precedence, used unless has value OptBoolean.DEFAULT (which is default value if property not included)
• In 2.19, plain required is still considered exactly like before (cannot really change without compatibility problems) -- although we could of course add MapperFeature to change it if we really really want to. I don't think we do?
• In 3.0, only value of required = true is considered (to mean property IS required) -- required = false is basically ignored, meaning same as isRequired = OptBoolean.DEFAULT.

So @k163377 I think Kotlin module 3.0 can implement better defaulting behavior.

I think there was some issue here too, related... will see if I can find it.