Use a std::shared_ptr<std::string> instead of std::string for the body_ member variable in Pistache::Http::Message (#814)

As a Pistache::Http::Message object is copied several times during the handling of a request this leads to a significant reduction in memory usage when uploading files through a http post.
Signed-off-by: Messiaen Arno <a.messiaen@televic.com>
Co-authored-by: Messiaen Arno <a.messiaen@televic.com>