Annotate core functional SPIs in SpEL with @FunctionalInterface

Prior to this commit, only the MethodFilter and ConstructorResolver
functional SPIs in the org.springframework.expression package were
annotated with @⁠FunctionalInterface.

For consistency, this commit designates each of the following
functional SPIs in that package as a @⁠FunctionalInterface as well.

- BeanResolver
- ConstructorExecutor
- MethodExecutor
- MethodResolver

Closes gh-32135

spring-expression/src/main/java/org/springframework/expression/BeanResolver.java

@@ -25,6 +25,7 @@ package org.springframework.expression;
  * @author Andy Clement
  * @since 3.0.3
  */
+@FunctionalInterface
 public interface BeanResolver {
 
 	/**

spring-expression/src/main/java/org/springframework/expression/ConstructorExecutor.java

@@ -37,6 +37,7 @@ package org.springframework.expression;
  * @see ConstructorResolver
  * @see MethodExecutor
  */
+@FunctionalInterface
 public interface ConstructorExecutor {
 
 	/**

spring-expression/src/main/java/org/springframework/expression/MethodExecutor.java

@@ -36,6 +36,7 @@ package org.springframework.expression;
  * @see MethodResolver
  * @see ConstructorExecutor
  */
+@FunctionalInterface
 public interface MethodExecutor {
 
 	/**

spring-expression/src/main/java/org/springframework/expression/MethodResolver.java

@@ -34,6 +34,7 @@ import org.springframework.lang.Nullable;
  * @see MethodExecutor
  * @see ConstructorResolver
  */
+@FunctionalInterface
 public interface MethodResolver {
 
 	/**