support `updated` parameter in packages listing

[chuckadams]

Prerequisite for this might be to support package listing at all -- I can't recall, I haven't put eyes on the code in a while 😕

We can support other search fields while we're at it, but as I've mentioned before, I don't want to encode every possible field in as url query string parameters. This one is special though, since it would be the basis for a poor man's cursor pagination. Works well enough for plc.directory and many other firehose-level streams, should be good enough for us. For proper pagination we'll want to support ordering after an id, and UUIDv7 does provide pretty good granularity there.

[toderash]

tag @bmack fyi - relates to the ?since= endpoint we've discussed, so AC can query Beacon sites for changes since the last successful update. This issue will need changes to Beacon, TER, and AC to support it.