MEDIUM: checks/http-fetch: Support htx prefetch from a check for HTTP samples

MEDIUM: checks/http-fetch: Support htx prefetch from a check for HTTP samples

Some HTTP sample fetches will be accessible from the context of a http-check
health check. Thus, the prefetch function responsible to return the HTX message
has been update to handle a check, in addition to a channel. Both cannot be used
at the same time. So there is no ambiguity.