MINOR: checks: Add a sample fetch to extract a block from the input check buffer

MINOR: checks: Add a sample fetch to extract a block from the input check buffer

It is now possible to extract information from the check input buffer using the
check.payload sample fetch. As req.payload or res.payload, an offset and a
length must be specified.

A new section has been added in the configuration manual. Now check sample
fetches will have to be documented under the section 7.3.7 (Fetching
health-check samples).