notify-addresses file format should support selecting notifications by arch

We have so many architectures now, often with diverse owners, that it's difficult to ensure the correct people are notified of image build failures: we either have to e.g. send architecture owners of ubuntu-server images email notifications for any build failures across 12+ architectures/subarchitectures, or leave them out and rely on ad-hoc forwarding of failure information.

Ideally we can select notifications by architecture, allowing architecture owners to identify patterns that will stand out more obviously to them.

I suggest an implementation of the notify-addresses file format that adds as a second field an 'architecture' field, consisting of a single whitespace-terminated string that supports globs. E.g. all existing entries can be mapped to 'edubuntu * $subscriber1 $subscriber2' and new entries of the form 'ubuntu-server riscv* $heinrich' can be added.

The production implementation of notify-addresses is in a private repository because the list of subscribers is not for public sharing, but access to this should not be required in order to develop an implementation in ubuntu-cdimage.