Add option to log URL requests

[qgib]

Author Name: Jeremy Palmer (@palmerj)
Original Redmine Issue: 18884
Affected QGIS version: 3.1(master)
Redmine category:network

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Add an new feature to log QGIS HTTP(s) URL calls in QGIS logging network tab. This should available as a debugging option in the network settings tab - much like the rendering settings tab for mapcanvas refresh. The feature should still be available even when QGIS is not built with debugging support.

The use case is to see what URLs QGIS sends when using webservices servers. Users can use feature this to help raise better issues.

Looking at https://github.com/qgis/QGIS/blob/master/src/core/qgsnetworkaccessmanager.cpp#L169 it doesn't even look like it's possible with Debug messages at the moment.

[alexbruy]

Implemented in master.