Teach relay-search about peeringdb's Network Type

In tpo/community/relays#73 where we are brainstorming network health tooling that would help the EFF university relay campaign, one of the ideas is to teach relay-search how to look up, list, and search by the 'Network Type' property that PeeringDB uses to categorize ASes.

Here's the summary from that ticket:

"""Use a database of (IP addresses or) ASes that map to educational institutions, so we can do a relay-search query of "show us all the relays that are on edu networks."

Right now I do that query by going to https://metrics.torproject.org/rs.html#aggregate/as and then manually skimming through the list for AS names that look edu related. Which is fine but there are 900 entries so doing a diff is hard and also I'm probably missing a few.

I talked to Arturo about this one at PETS, and he pointed out that PeeringDB already categorizes its ASes into network types. For example, AS3 (MIT) is listed as "Network Type: Educational/Research": https://www.peeringdb.com/net/7092

So I think the next step there would be to teach relay-search how to look up and display the Network Type for the AS. And then the step after that would be to let us search relays by Network Type."""