Support custom callouts

[Nicolapps]

https://developer.apple.com/documentation/mapkitjs/mapkit/annotation/2973819-callout

[nikischin]

I was working on this here: https://github.com/nikischin/mapkit-react/tree/feature/customCallout, though not yet ready to merge. If you would like to continue on this, this would be great, as I am facing some challenges.

[nikischin]

Implemented with #55