Spring Create immutable MultiValueMap wrapper

Currently, CollectionUtils::unmodifiableMultiValueMap returns an (unmodifiable) copy of the target map, and therefore allocates extra memory. We cannot use the Collections.unmodifiableMap wrapper in said method, because each value of the map should be immutable as well, and thus needs a Collections.unmodifiableList wrapper.

We should create a UnmodifiableMultiValueMap wrapper, similar to the JDK Collections, that wraps entries and values with immutable counterparts where needed.