MINOR: sample: Add converters to parse FIX messages

MINOR: sample: Add converters to parse FIX messages

This patch implements a couple of converters to validate and extract tag value
from a FIX (Financial Information eXchange) message. The validation consists in
a few checks such as mandatory fields and checksum computation. The extraction
can get any tag value based on a tag string or tag id.

This patch requires the istend() function. Thus it depends on "MINOR: ist: Add
istend() function to return a pointer to the end of the string".

Reviewed and Fixed by Christopher Faulet <cfaulet@haproxy.com>