Update bandwidth file headers parser to spec version 1.4.0

The bandwidth file spec version 1.4.0 (https://gitweb.torproject.org/torspec.git/tree/bandwidth-file-spec.txt#n63) adds new KeyValues. An example: http://199.58.81.140/tor/status-vote/next/bandwidth. Probably stem should update the headers parser with them, as part of #21378 (moved).

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