MINOR: httpclient: implement a simple HTTP Client API

This commit implements a very simple HTTP Client API.

A client can be operated by several functions:

    - httpclient_new(), httpclient_destroy(): create
      and destroy the struct httpclient instance.

    - httpclient_req_gen(): generate a complete HTX request using the
      the absolute URL, the method and a list of headers. This request
      is complete and sets the HTX End of Message flag. This is limited
      to small request we don't need a body.

    - httpclient_start() fill a sockaddr storage with a IP extracted
      from the URL (it cannot resolve an fqdm for now), start the
      applet. It also stores the ptr of the caller which could be an
      appctx or something else.

   - hc->ops contains a list of callbacks used by the
     HTTPClient, they should be filled manually after an
     httpclient_new():

        * res_stline(): the client received a start line, its content
          will be stored in hc->res.vsn, hc->res.status, hc->res.reason

        * res_headers(): the client received headers, they are stored in
          hc->res.hdrs.

        * res_payload(): the client received some payload data, they are
        stored in the hc->res.buf buffer and could be extracted with the
        httpclient_res_xfer() function, which takes a destination buffer
        as a parameter

        * res_end(): this callback is called once we finished to receive
        the response.
4 files changed