MINOR: httpclient/cli: add --htx option

MINOR: httpclient/cli: add --htx option

Use the new HTTPCLIENT_O_RES_HTX flag when using the CLI httpclient with
--htx.

It allows to process directly the response in HTX, then the htx_dump()
function is used to display a debug output.

Example:

echo "httpclient --htx GET https://haproxy.org" | socat /tmp/haproxy.sock
 htx=0x79fd72a2e200(size=16336,data=139,used=6,wrap=NO,flags=0x00000010,extra=0,first=0,head=0,tail=5,tail_addr=139,head_addr=0,end_addr=0)
		[0] type=HTX_BLK_RES_SL    - size=31     - addr=0     	HTTP/2.0 301
		[1] type=HTX_BLK_HDR       - size=15     - addr=31    	content-length: 0
		[2] type=HTX_BLK_HDR       - size=32     - addr=46    	location: https://www.haproxy.org/
		[3] type=HTX_BLK_HDR       - size=25     - addr=78    	alt-svc: h3=":443"; ma=3600
		[4] type=HTX_BLK_HDR       - size=35     - addr=103   	set-cookie: served=2:TLSv1.3+TCP:IPv4
		[5] type=HTX_BLK_EOH       - size=1      - addr=138   	<empty>