New head tracker

[mempirate]

Context

We're using a head tracker in all 3 components (sidecar, RPC, registry). We should make an effort to consolidate all of them into a single fault-tolerant, consistent, reorg aware head tracker, that can be customized based on the needs of the user. This can be extracted to a separate crate: bolt-common.

Example customization: some users only need consensus URL for tracking slots, others need more info like EL block number¹, others even more like EL block header data, or even the whole block with all the populated transactions. Can use a combination of builder and type state pattern here (type state because ideally you want any components to define at compile time what your head tracker needs to support).

First step would be identifying all requirements across the components that currently use it.

Requirements

• Sync-aware: https://ethereum.github.io/beacon-APIs/#/Node/getSyncingStatus (sidecar, rpc, registry)
• Unconditional epoch transition signalling, even with missed slots (registry)

Footnotes

1. In the registry, we use the payload_attributes event stream on the beacon-API, which contains EL block number as well. ↩