Adds a method to zoom the video capture device using an optional rate

[dsmurfin]

This is only available on iOS

[YuAo]

• Can you make this a separate extension like Camera+FocusExposure.swift.
• It's better to just ignore the call when no video device is attached to match the current behavior of related setFocusExposurePointOfInterest, setExposureTargetBias methods, though I think throwing may be a good idea however let's just match the behavior for now.
• How about clamping the zoomFactor to the supported range?

[dsmurfin]

This makes much more sense, I'll implement that way.

Clamping is very sensible. At the moment I'm doing this on my side before calling into VideoIO, but we should do this here.

[dsmurfin]

@YuAo this PR represents changes as discussed previously. The failing tests are all testPlayerVideoOutput_iOS which does not test any of the code impacted by this PR, and which I believe may need a longer timeout due to some environmental changes.