Revisit nullability annotations

This commit introduces the following changes.

1) It adds a new Spring @NonNull annotation which allows to apply
@NonNullApi semantic on a specific element, like @Nullable does.
Combined with @Nullable, it allows partial null-safety support when
package granularity is too broad.

2) @Nullable and @NonNull can apply to ElementType.TYPE_USE in order
to be used on generic type arguments (SPR-15942).

3) Annotations does not apply to ElementType.TYPE_PARAMETER anymore
since it is not supported yet (applicability for such use case is
controversial and need to be discussed).

4) @NonNullApi does not apply to ElementType.FIELD anymore since in a
lot of use cases (private, protected) it is not part for the public API
+ its usage should remain opt-in. A dedicated @NonNullFields annotation
has been added in order to set fields default to non-nullable.

5) Updated Javadoc and reference documentation.

Issue: SPR-15756

spring-aop/src/main/java/org/springframework/aop/aspectj/annotation/package-info.java

@@ -4,6 +4,8 @@
  * <p>Normally to be used through an AspectJAutoProxyCreator rather than directly.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.aop.aspectj.annotation;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-aop/src/main/java/org/springframework/aop/aspectj/autoproxy/package-info.java

@@ -3,6 +3,8 @@
  * Support for AspectJ annotation aspects resides in the "aspectj.annotation" package.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.aop.aspectj.autoproxy;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-aop/src/main/java/org/springframework/aop/aspectj/package-info.java

@@ -9,6 +9,8 @@
  * functionality, with consistent semantics, with the proxy-based Spring AOP framework.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.aop.aspectj;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-aop/src/main/java/org/springframework/aop/config/package-info.java

@@ -3,6 +3,8 @@
  * with XML schema being the primary configuration format.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.aop.config;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-aop/src/main/java/org/springframework/aop/framework/adapter/package-info.java

@@ -10,6 +10,8 @@
  * <p>These adapters do not depend on any other Spring framework classes to allow such usage.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.aop.framework.adapter;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-aop/src/main/java/org/springframework/aop/framework/autoproxy/package-info.java

@@ -10,6 +10,8 @@
  * Post-processors can be explicitly registered on a ConfigurableBeanFactory instead.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.aop.framework.autoproxy;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-aop/src/main/java/org/springframework/aop/framework/package-info.java

@@ -13,6 +13,8 @@
  * ProxyFactory class.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.aop.framework;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-aop/src/main/java/org/springframework/aop/interceptor/package-info.java

@@ -4,6 +4,8 @@
  * functionality packages, like "transaction" and "orm".
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.aop.interceptor;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-aop/src/main/java/org/springframework/aop/package-info.java

@@ -18,6 +18,8 @@
  * integrated with the Spring IoC container.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.aop;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-aop/src/main/java/org/springframework/aop/scope/package-info.java

@@ -2,6 +2,8 @@
  * Support for AOP-based scoping of target objects, with configurable backend.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.aop.scope;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-aop/src/main/java/org/springframework/aop/support/annotation/package-info.java

@@ -2,6 +2,8 @@
  * Annotation support for AOP pointcuts.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.aop.support.annotation;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-aop/src/main/java/org/springframework/aop/support/package-info.java

@@ -2,6 +2,8 @@
  * Convenience classes for using Spring's AOP API.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.aop.support;
 
-import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;