observer: add CCADB CRL prober

[jsha]

Prior work: letsencrypt/crl-monitor#88

Fixes #8618

[aarongable]

I have a few minor comments about the code, but nothing big.

My major concern is that this simply doesn't feel like a prober. It's doing so much work, and has so many opportunities for failure, all of which get wrapped up in big accumulated errors. In my mind, the ideal prober is short, simple, only emits metrics (not meaningful logs), and we have alerts for every single metric it exposes. I understand that it is useful for this to live within boulder-observer, so we don't have to deploy and monitor Yet Another Component, but I'm registering my moderate discomfort with the complexity here.