MINOR: proxy/http_ext: introduce proxy forwarded option

MINOR: proxy/http_ext: introduce proxy forwarded option

Introducing http_ext class for http extension related work that
doesn't fit into existing http classes.

HTTP extension "forwarded", introduced with 7239 RFC is now supported
by haproxy.

The option supports various modes from simple to complex usages involving
custom sample expressions.

  Examples :

    # Those servers want the ip address and protocol of the client request
    # Resulting header would look like this:
    #   forwarded: proto=http;for=127.0.0.1
    backend www_default
        mode http
        option forwarded
        #equivalent to: option forwarded proto for

    # Those servers want the requested host and hashed client ip address
    # as well as client source port (you should use seed for xxh32 if ensuring
    # ip privacy is a concern)
    # Resulting header would look like this:
    #   forwarded: host="haproxy.org";for="_000000007F2F367E:60138"
    backend www_host
        mode http
        option forwarded host for-expr src,xxh32,hex for_port

    # Those servers want custom data in host, for and by parameters
    # Resulting header would look like this:
    #   forwarded: host="host.com";by=_haproxy;for="[::1]:10"
    backend www_custom
        mode http
        option forwarded host-expr str(host.com) by-expr str(_haproxy) for for_port-expr int(10)

    # Those servers want random 'for' obfuscated identifiers for request
    # tracing purposes while protecting sensitive IP information
    # Resulting header would look like this:
    #   forwarded: for=_000000002B1F4D63
    backend www_for_hide
        mode http
        option forwarded for-expr rand,hex

By default (no argument provided), forwarded option will try to mimic
x-forward-for common setups (source client ip address + source protocol)

The option is not available for frontends.
no option forwarded is supported.

More info about 7239 RFC here: https://www.rfc-editor.org/rfc/rfc7239.html

More info about the feature in doc/configuration.txt

This should address feature request GH #575

Depends on:
  - "MINOR: http_htx: add http_append_header() to append value to header"
  - "MINOR: sample: add ARGC_OPT"
  - "MINOR: proxy: introduce http only options"