Add new syntax for adding and removing attributes from record fields:
redef RecordType$field_name += { &log };
redef RecordType$field_name -= { &log };
For now this only allowed for the &log attribute as the semantics are clear.
For &default and &optional the semantics aren't obvious and no use-cases have
been identified where those would make sense to change.
This enables a mechanism to add potentially interesting fields to the typical
Info records in base scripts, but letting users opt-into actually including
them into their log. At the same time, users that find specific fields in a
standard log uninteresting can opt-out without using `Log::Filter$exclude`
which can be difficult to use correctly. Patching or forking external packages
to remove columns from a log can also be avoided with this mechanism.
Closes#2000.
