Add support for annotating targets and hostnames

[Arturo Filastò]

It would be nice to add annotations that we can update over time related to hostnames.

Things that I would like to be able to say are flags like if the hostname hosts a domain name parking page, if the hostname is expired or any other feature that's relevant to the analysis.

See also:

• ooni/probe#1826
• ooni/probe#1745

Relevant issues:

• ooni/backend#177
• ooni/backend#670
• ooni/backend#506
• ooni/backend#570
• ooni/backend#609
• ooni/backend#611

[Arturo Filastò]

Sharing additional notes from slack:

On the topic of parked domains, there is this paper that documents a methodology for automatically detecting such domains: https://www.ndss-symposium.org/ndss2015/ndss-2015-programme/parking-sensors-analyzing-and-detecting-parked-domains/.
I think for a basic first pass we don’t even have to implement their ML based approach, but just use the list of the most common nameservers used by domain parking services and filter them out based on that (there is such a list in the paper).