hicn_plugin is currently a dependency of libhicntrasnport, although libtransport only uses few hicn_plugin header files and it is not linked against hicn_plugin.

This patch allows libtransport to be built independently of hicn_plugin.

Main updates:

• The files hicn.api.h, hicn.api_types.h, hicn.api_enum.h, hicn.api.vapi.h and hicn.api.vapi.hpp are now generated at configuration time, so that they are immediately available for sibprojects using them (libtransport)
• Libtransport does not have to wait for the compilation of hicn_plugin anymore, since the required files are immediately avaiable upon project configuration
• The dependency hicn_plugin can be removed from libtransport.