Create bandwidth authority specification

We have:

• https://research.torproject.org/techreports/torflow-2009-08-07.pdf
• https://gitlab.torproject.org/tpo/network-health/torflow/-/blob/main/NetworkScanners/BwAuthority/README.spec.txt
• https://gitlab.torproject.org/tpo/network-health/torflow/-/blob/main/NetworkScanners/BwAuthority/README.BwAuthorities
• https://gitlab.torproject.org/tpo/network-health/torflow/-/blob/main/README
• https://gitlab.torproject.org/tpo/core/torspec/-/blob/main/bandwidth-file-spec.txt

We should create a single "bandwidth authority spec" including data from those documents and the current implementations.