Switch to structured logging

[phbnf]

TesseraCT uses klog and implements some abstraction with requestLog. Let's switch to structured logging, probably with slog, to have better logs. It should also make them more usable in Log Explorer. At the moment on GCP, I have not found a way to merge logs of all VMs within a Managed Instance Group. I'm hoping that structured logs would help solve this?

[mhutchinson]

https://github.com/kubernetes/community/blob/master/contributors/devel/sig-instrumentation/logging.md - klog already supports structured logging. What structures are required? It seems to be key-value pairs, so we probably need a bunch of keys we'd like to see populated across the board.