Add option to get notification for bridge events (like bridge is down)

At the last weekend's relay operator meetup we got asked to consider bridges as well to be eligible for inclusion in our weather monitoring system. That's in principle doable, however, using Onionoo for that might not be the right choice as e.g. some bridges have firewalled their ORPort to avoid censorship efforts. Thus, Onionoo might falsely report those bridges are down while they are actually fine and the bridge authority, Serge, can't just connect to them.

We could use bridgestrap output instead to get a clearer picture. Not sure how much of time-granularity we could offer in that case. Probably not a notify-me-after-my-bridge-is-down-for-an-hour.

Alternatively, if that's not sufficient we could think about running our own bridgestrap client for the bridges to monitor.

/cc @meskio from the a-c team