Spring Introduce MapAccessor in SpEL and deprecate existing implementation in spring-context

The MapAccessor for SpEL currently resides in the org.springframework.context.expression package in the spring-context module; however, it arguably should reside in the spring-expression module so that it can be used whenever SpEL is used (without requiring spring-context).

We should therefore officially deprecate the MapAccessor in spring-context for removal in favor of a new copy of the implementation in the org.springframework.expression.spel.support package in spring-expression.