Add KeyValue with the name of the bandwidth authority

We've mentioned this several times, when searching for bugs: it would be easier to find out from which bwauth an archived Bandwidth File has been generated if it would be included in the file itself.

Currently it's only possible to correlate the hash of the file with the dirauth in the vote. Or guess it by the size of the file and the time it has been published, but it's just a guess.

This has also be commented in tpo/network-health/metrics/collector#33061 (closed).

It also have more sense than having an scanner nickname.