Improve/document the observability story

[saolof]

qstat lets you query the current state of pbs jobs, but it is not ideal for implementing the observer pattern instead of polling the current state or for getting the full history of state changes, which would be nice to have when trying to integrate pbs with other parts of a toolchain, like getting a live notification that a job has been killed.

The ideal end goal would be to be able to stream a change log to databases/kafka/loki. I don't quite expect pbs to communicate with these by itself, but it would be nice to have some documented interface to be able to write monitoring processes that do this, at any level of abstraction from reading a log files dumped to disk to having debezium use the logical replication functionality of the pbs postgres database