Use events API to eager send attestations

[hopinheimer]

addresses:

• Use the events api to attest on a head event #7820

[michaelsproul]

@hopinheimer We realised that the fallback to the other BNs wouldn't work due to the main loop waiting for the next slot on failure. So we've refactored quite substantially so that:

• On head event: request attestation data from that BN and check that it matches the expected head.
• If that works, we have our attestation data, proceed as before.
• If that doesn't work, wait until attestation deadline then run first_success on all nodes to get attestation data.

These changes are in this commit:

• 0e9dab3

In the process of this change I deleted first_success_from_index and adapted its tests (simpler now). I also made spawn_attestation_tasks async so we can just fetch the attestation data inline, and adjusted some of the math to account for this (avoiding calculating duration_to_next_slot early in case attestation data production takes substantial time (expected), and being explicit about which slot we are waiting for).

[mergify]

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[michaelsproul]

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✅ The pull request has been merged at 0e9dab3

This pull request spent 33 minutes 11 seconds in the queue, including 30 minutes 53 seconds running CI.
The checks were run on draft #8745.

Required conditions to merge

• check-success=local-testnet-success
• check-success=test-suite-success

[michaelsproul]

@mergify dequeue

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[michaelsproul]

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