Fix convex function ignoring given properties

[blchelle]

Background

Using the convex function to combine a feature collection returns a newly constructed polygon with missing properties. This is crucial if the caller of the function attempts to pass additional properties to the end polygon.

PR includes a test to test this scenario and improved typing

Resolve #2762

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[kareemjeiroudi]

Thanks @blchelle for taking over. I'll close this PR #2765, as soon as yours is merged 👍.