Spec: split out forDebuggingOnly fields from generated bid

...into new type =bid debug reporting info=. This is in preparation
for multibid, since that would have multiple bids corresponding to
a single struct of debug reporting URLs [1]. As a bonus,
this permits the removal of slightly awkward "no bid" field on
generated bid.

[1] So =bid debug reporting info= will ultimately have a /set/ of
generated bid ids, rather than id-or-null.