Implement bandwidth file parser and formater

After #21377 (moved) is implemented, the bandwidth authorities will start publishing the bandwidth files that they have used in their votes and these will be stored by CollectTor. This is currently implemented in sbws, but it's possible that we still need to change the format of the bandwidth file. As discussed in #28684 (moved), it would be a problem to parse files already archived if the format of the bandwidth file changes, so it would be better if this implemented in stem.