Allow setting the GPS location accuracy in iOS/macOS

Currently, it seems that the GPS location accuracy is hardcoded to `kCLLocationAccuracyBest`.

https://codereview.qt-project.org/gitweb?p=qt/qtlocation.git;a=blob;f=src/plugins/position/corelocation/qgeopositioninfosource_cl.mm;h=2d412138364e24f4831ac871de0d7ec363672133;hb=refs/heads/5.15#l198

This yields suboptimal results for many use cases, as frequently the user will want to use lower resolution and decrease battery usage, , or increase resolution for maximal effect.

Request: the developer should be able to set the option without having to recompile Qt source.