Add `published` line to `@type bridge-network-status` documents.

This modifies the format of the bridge networkstatus documents produced
by the BridgeAuth. The new format adds a `published` line to the header
of the file, before the `flag-thresholds` line. This is done once per
networkstatus file that is written. The timestamp in the `published`
line is the standard ISO 8601 format (with a space as the separator)
used throughout the rest of tor.

 * FIXES #12951 https://bugs.torproject.org/12951