[ios] Store annotation tags for MGLMultiPoint annotations in the map

[boundsj]

This fixes an issue where, if a multipoint annotation was added, native annotation view update logic would crash. Adding the annotation tag for multipoints, too, avoids the out of range exception.

cc @1ec5 @incanus

[mention-bot]

@boundsj, thanks for your PR! By analyzing the history of the files in this pull request, we identified @1ec5, @incanus and @friedbunny to be potential reviewers.

[boundsj]

Sorry @1ec5, I just realized I missed adding the check for multipoints. Does not really change anything but I added it in f0039fd for consistency.