enh(navigation): openvslam vocabulary is quite heavy

Description

Only download vocabulary file when starting Openvslam module.

• Download has been removed from CMakeLists.txt
• A function to download files from server to host has been created in io_http using libcurl with associated unit tests
• The download function is called by the Plugin::start of navigation::openvslam to dowload file if it doesn't already exists

Closes #900 (closed)

How to test it?

1. Build Sight by enabling Openvslam in CMake options
2. make sure the orb_voc.dbow2 isn't already present in share/sight/module_navigation_openvslam
3. Launch ExOpenvslam
4. The file should be downloaded at start