Copy the signing_key_cert field into signed_descriptor_t

We need this field to be in signed_descriptor_t so that
routerinfo_incompatible_with_extrainfo can work correctly (#17150).
But I don't want to move it completely in this patch, since a great
deal of the code that messes with it has been in flux since 0.2.7,
when this ticket was opened.  I should open another ticket about
removing the field from routerinfo_t and extrainfo_t later on.

This patch fixes no actual behavior.