MINOR: http_htx: add http_append_header() to append value to header

MINOR: http_htx: add http_append_header() to append value to header

Calling this function as an alternative to http_replace_header_value()
to append a new value to existing header instead of replacing the whole
header content.

If the header already contains one or multiple values: a ',' is automatically
appended before the new value.

This function is not meant for prepending (providing empty ctx value),
in which case we should consider implementing dedicated prepend
alternative function.