Observability support

Things that we could do:
- Keep counters that track communication statistics between workers, like the number of RPC calls and the amount of data sent/received.
- Expose a hook API so packages can register functions that will be called on sending/receiving data. Not sure about the performance implications of this.
- Read-only API to check the status of a worker, the time of last contact, and state of worker-to-worker connections.

Use cases:
- Tracking network usage between workers on HPC clusters
- Better support for profiling tools like Extrae (https://github.com/JuliaParallel/DistributedNext.jl/issues/1#issuecomment-2437194907)