[ios] Add annotation view scaling (#5085)

This adds an option to MGLAnnotationView so that a developer can control
how their annotation views scale in size as they approach the top of
the map view (along the Y axis). The size change is also affected by
the pitch (tilt) of the map view. If there is no tilt then scaling
is not performed. As the map is tilted scaling is gradually applied.

This turns the MGLAnnotationView implementation into an Objective C++
class so that SDK constants from C++ files can be used.

The new behavior is enabled by default to maintain consistency with
annotations backed by GL sprites. The MBXViewController redundantly
sets the new property as an illustration of use.

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