wip: add support for ingesting events via pubsub

[bhearsum]

When completed, this should add support for consuming pubsub messages from GCS product delivery buckets. I suggest we continuing running both this and SQS at first, and then:

• Switch off buildhub uploads to SQS
• Verify that buildhub still gets all the data it needs
• Turn off & rip out SQS support in buildhub

This actually should be quite easy - the pubsub API is pretty straightforward and compatible with the current architecture. The tricky part is that buildhub is woefully out of date, which makes adding new dependencies terribly difficult. The tests also don't seem to run without a postgres database being available, which makes the development cycle annoying and more difficult.

I suggest we try to bring buildhub up to date before tackling this any further.

[bhearsum]

#1045 and #1046 for bringing the Python side of things up to speed, #1047 for test improvements.

[bhearsum]

I also spun the Python 3.12 upgrade into #1048