Anti-slashing via VC shutdown on SSE events

[paulhauner]

Description

I’ve seen third party tooling that monitors a set of validators and kills the VC process if there’s a slashing. I believe we could implement similar functionality ourselves.

I propose that the VC listens for attester and proposer slashing events via the BeaconAPI SSE stream and shuts down if it detects a slashing on a key that it manages.

I’d expect the VC to write a file to disk that prevents further starts. The user would need to manually delete that file to restart the VC. This functionality is the same as I suggested in #6665, so we should coordinate there.

[paulhauner]

Issue 6666 🤘