move using to avhttp namespace

Signed-off-by: Jack <jack.wgm@gmail.com>
change specified flag && add async access http resource example

Signed-off-by: Jack <jack.wgm@gmail.com>
improve example

Signed-off-by: Jack <jack.wgm@gmail.com>
...

Signed-off-by: Jack <jack.wgm@gmail.com>
add socket_type.hpp

Signed-off-by: Jack <jack.wgm@gmail.com>
add ssl implementation

Signed-off-by: Jack <jack.wgm@gmail.com>
improve http_stream && implement https

Signed-off-by: Jack <jack.wgm@gmail.com>