Introduce Nullability API

[mp911de]

We now provide a dedicated API to introspect nullability rules. The new API supports Spring, JSR-305, and JSpecify.

Nullability nullability = Nullability.forMethodReturnType(method);
nullability.isDeclared();
nullability.isNullable();
nullability.isNonNull();

// introspect multiple elements for their nullability in the scope of a class/package.
Nullability.Introspector introspector = Nullability.introspect(NonNullOnPackage.class);
Nullability nullability = introspector.forReturnType(method);

The previous NullableUtils API is now deprecated as its implementation would not support a pluggable model of NullabilityProvider.

Related to #3100

[mp911de]

Paging @sdeleuze

[odrotbohm]

Should this rather live in Spring Framework?

[sdeleuze]

If we added more nullabilty runtime API, it would indeed be worth to considering it for inclusion in Spring Framework, but in terms of timeframe it would be more towards next year. As shared with @mp911de, nullabilty is a complex and nuanced topic with a lot of implications, and there are some ongoing discussions with JSpecify working group on potential evolution that could impact how Spring supports it, as well as some future implication on JDK side. With @jhoeller, in terms of timeline we suggest to wait for related portfolio-wide discussions to evaluate what could go on Framework side before merging this PR.