MINOR: mux-h1: Add support of the debug string for logs

MINOR: mux-h1: Add support of the debug string for logs

Now it is possible to have info about front and back H1 multiplexer. For instance:

    <134>Oct 22 18:10:46 haproxy[3841864]: 127.0.0.1:44280 [22/Oct/2024:18:10:43.265] front-http back-http/www 0/0/-1/-1/3082 503 217 - - SC-- 1/1/0/0/3 0/0 "GET / HTTP/1.1" fs=< h1s=0x13b6f10 h1s.flg=0x14010 .sd.flg=0x50404601 .req.state=MSG_DONE .res.state=MSG_DONE .meth=GET status=503 .sd.flg=0x50404601 .sc.flg=0x00034482 .sc.app=0x11e4c30 .subs=(nil) h1c.flg=0x0 .sub=0 .ibuf
=0@(nil)+0/0 .obuf=0@(nil)+0/0 .task=0x1337d10 .exp=<NEVER> conn.flg=0x80000300> bs=< h1s=0x13bb400 h1s.flg=0x100010 .sd.flg=0x10400001 .req.state=MSG_RQBEFORE .res.state=MSG_RPBEFORE .meth=UNKNOWN status=0 .sd.flg=0x10400001 .sc.flg=0x0003c007 .sc.app=0x11e4c30 .subs=(nil) h1c.flg=0x80000000 .sub=0 .ibuf=0@(nil)+0/0 .obuf=0@(nil)+0/0 .task=0x12ba610 .exp=<NEVER> conn.flg=0x5c0300>

The have this log message, the log-format must be set to:

  log-format "$HAPROXY_HTTP_LOG_FMT fs=<%[fs.debug_str]> bs=<%[bs.debug_str]>"

(cherry picked from commit ce314cfb395a407484e1b37d76e8af8007c1ff4c)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>