Support using sift from main repository without using nonfree components

[traversaro]

If the only class used from the nonfree component of OpenCV are SIFT, note that due to the fact that the SIFT patent expired, since OpenCV 4.4.0 SIFT are part of OpenCV main (see https://github.com/opencv/opencv/wiki/ChangeLog#version440 ).

This mean that for example from Ubuntu 22.04 (that has OpenCV 4.5.*: https://repology.org/project/opencv/versions ), it should be possible to compile stereo-vision using opencv installed via apt, avoiding problems like https://github.com/robotology/stereo-vision/issues/33 .