Add bridge authority fingerprint to sanitized bridge network statuses

When I looked through existing code that uses methods from DescriptorFile other than DescriptorFile#getDescriptors() for #22141 (moved), I spotted this code in metrics-web:

          if (descriptorFile.getFileName().contains(
              "4A0CCD2DDC7995083D73F5D667100C8A5831F16D")) {
            authority = "Tonga";
          } else if (descriptorFile.getFileName().contains(
              "1D8F3A91C37C5D1C4C19B1AD1D0CFBE8BF72D8E1")) {
            authority = "Bifroest";
          }

Not beautiful, I know, but that's not the point. The point is that we need to parse the bridge authority fingerprint from the file name, because it's not contained in the descriptor itself. That seems bad, and relatively easy to fix.

Right now, the header of a sanitized bridge network status looks like this:

@type bridge-network-status 1.1
published 2017-05-09 07:57:32
flag-thresholds stable-uptime=1075980 stable-mtbf=2384868 fast-speed=21000 guard-wfu=98.000% guard-tk=691200 guard-bw-inc-exits=318000 guard-bw-exc-exits=318000 enough-mtbf=1 ignoring-advertised-bws=0

We could easily include a "fingerprint" line there, like this:

@type bridge-network-status 1.2
published 2017-05-09 07:57:32
flag-thresholds stable-uptime=1075980 stable-mtbf=2384868 fast-speed=21000 guard-wfu=98.000% guard-tk=691200 guard-bw-inc-exits=318000 guard-bw-exc-exits=318000 enough-mtbf=1 ignoring-advertised-bws=0
fingerprint 1D8F3A91C37C5D1C4C19B1AD1D0CFBE8BF72D8E1

Ideally, we'd modify the bridge authority code to include this line, too, and then go through archived bridge network statuses and retroactively put it in.

This is somewhat related to #12951 (moved) when we added a "published" line to bridge network statuses in a similar way.

I'm copying isis to hear what they think about putting in a "fingerprint" line.