Constellation: filter firehose by collection NSID or NSID prefix #6

Right now constellation always indexes everything in the firehose. This is good for the main public instance, but self-hosters might be interested in only a subset of network backlinks.

Add some configuration, probably as a config file, which would be specified as a command line argument, and limit what links constellation will index.

• would be nice to have both include and exclude rules
• it's worth considering how filters might to either the subject of the link vs source of the link

for source filtering, we can probably lean on jetstream directly to apply filtering before it even gets to us. for subject filtering we'd need to receive all links and apply the filter after extraction.