GML: autodiscover all XML attributes as OGR fields when creating .gfs file if GML_ATTRIBUTES_TO_OGR_FIELDS is set to YES (#5418)

git-svn-id: https://svn.osgeo.org/gdal/trunk@27120 f0d54148-0727-0410-94bb-9a71ac55c965
Merge branch 'trunk' into trunk_travis_macosx