This commit adds a new andWithPrefix(String, FieldDescriptor[]) method
to both RequestFieldsSnippet and ResponseFieldsSnippet. It can be
used to add descriptors to an existing snippet, applying the given
prefix to the additional descriptors as it does so. This allows the
descriptors for a portion of a payload to be created once and then
reused, irrespective of where in the payload the portion appears.
Closes gh-221