Spring Annotate core functional SPIs in SpEL with @FunctionalInterface

In the org.springframework.expression package, we have the following functional interfaces.

• BeanResolver
• ConstructorExecutor
• ConstructorResolver
• MethodExecutor
• MethodFilter
• MethodResolver

However, currently only MethodFilter and ConstructorResolver are actually annotated with @FunctionalInterface.

For consistency, we should annotate each of the remaining functional SPIs with @FunctionalInterface.