Add parsing support for bridge network statuses to Stem

When parsing bridge descriptor tarballs, Stem doesn't like the contained network statuses:

ParsingFailure: Unrecognized metrics descriptor format. type: 'bridge-network-status', version: '1.0'

This format isn't as explicitly defined as the formats of other network statuses. Bridge network statuses start with a "@type bridge-network-status 1.0" line, followed by a "published" line, followed by router status entries, not followed by any kind of footer. This format is produced by metrics-db as part of the sanitizing process. If we were to specify it more explicitly, we should add a new section after the sanitized bridge descriptor specification.