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-expression/src/main/java/org/springframework/expression/common/package-info.java

@@ -2,6 +2,8 @@
  * Common utility classes behind the <em>Spring Expression Language</em>.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.expression.common;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

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

@@ -2,6 +2,8 @@
  * Core abstractions behind the <em>Spring Expression Language</em>.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.expression;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-expression/src/main/java/org/springframework/expression/spel/ast/package-info.java

@@ -2,6 +2,8 @@
  * SpEL's abstract syntax tree.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.expression.spel.ast;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-expression/src/main/java/org/springframework/expression/spel/package-info.java

@@ -2,6 +2,8 @@
  * SpEL's central implementation package.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.expression.spel;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

spring-expression/src/main/java/org/springframework/expression/spel/standard/package-info.java

@@ -2,6 +2,8 @@
  * SpEL's standard parser implementation.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.expression.spel.standard;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;

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

@@ -2,6 +2,8 @@
  * SpEL's default implementations for various core abstractions.
  */
 @NonNullApi
+@NonNullFields
 package org.springframework.expression.spel.support;
 
 import org.springframework.lang.NonNullApi;
+import org.springframework.lang.NonNullFields;