MEDIUM: http-fetch: Rework how HTTP message version is retrieved

MEDIUM: http-fetch: Rework how HTTP message version is retrieved

Thanks to previous patches, we can now rely on the version stored in the
http_msg structure to get the request or the response version.

"req.ver" and "res.ver" sample fetch functions returns the string
representation of the version, without the prefix, so "<major>.<minor>", but
only if the version is valid. For the response, "res.ver" may be added from
a health-check context, in that case, the HTX message is used.

"capture.req.ver" and "capture.res.ver" does the same but the "HTTP/" prefix
is added to the result. And "capture.res.ver" cannot be called from a
health-check.

To ease the version formatting and avoid code duplication, an helper
function was added. So these samples are now relying on "get_msg_version()".