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 👍.

[blchelle]

@smallsaucepan would you be able to take a look at this PR when you have a chance?

[blchelle]

@smallsaucepan do you need anything from me to help get this PR merged?

[smallsaucepan]

Thanks for putting this together. Couple of small things to change / check.