mechanism for jefe to deliver ereports to packrat

[hawkw]

PR #2126 adds the initial implementation for ereport aggregation in the packrat task, and for ereport ingestion over the management network via the snitch task. However, one thing that this branch does not implement is a mechanism for ereports from jefe to be delivered to packrat: because packrat receives ereports via an IPC call, they may only be delivered by lower-priority tasks.

In order to generate ereports for events like "a task faulted" and "a crash dump is available to collect", we need a way for jefe to produce ereports. The likely way to implement this is to say "don't call me, I'll call you", and give jefe an IPC method that packrat calls into to receive an ereport. We can notify packrat that it should call this IPC by adding a notification from jefe to packrat that says "hey i have an ereport."

[hawkw]

One open design question here is whether we should do this by having jefe encode a CBOR ereport (the way that other tasks that generate ereports do), or if we should add an IPC that's specifically for "get last task fault" that returns more structured Rust types. Generating a CBOR ereport in packrat is more flexible, but requires putting CBOR encoding code in the supervisor, which might not be desirable. A generalized "give me data about recent faults" interface could also be usable by other subsystems beyond the ereport subsystem...