Use a single Supplier instance inside a configured MultiMapBuilder

[CodingFabian]

fixes #5673

[CodingFabian]

This change required a bit of generics magic, since previously the suppliers got a type assigned that was dependent on the V type of the assigned variable (from the return type of the build signature)

I followed the pattern already existing for EnumSet to derive V from V0, and that does compile correctly with the forced V0 -> V casts

generally speaking: the type here doesn't matter. the Suppliers will always create empty backing collections which can be of any type at compile time and runtime.

if there is a better way to do this, I appreciate the insight

[CodingFabian]

If consistency is preferred, I think we can rework LinkedListSupplier supplier to also use an instance per builder, instead of the lazily initialized global one.

private enum LinkedListSupplier implements Supplier<List<?>> {
    INSTANCE;

    public static <V extends @Nullable Object> Supplier<List<V>> instance() {
      // Each call generates a fresh LinkedList, which can serve as a List<V> for any V.
      @SuppressWarnings({"rawtypes", "unchecked"})
      Supplier<List<V>> result = (Supplier) INSTANCE;
      return result;
    }