Decouple builder registrations from proposer flow

[KaloyanTanev]

🎯 Problem to be solved

Currently we do wait for successful builder registrations at slot 0 before triggering the start of the proposer duty.

This is incorrect and causes only slowdowns. Builder registrations are usually saved in the DB of the block builders, so broadcasting them each and every epoch is more of a sanity check, rather than a requirement. A builder will have the builder registration saved in its DB for months/years if there isn't some issues with its infrastructure.

🛠️ Proposed solution

1. [MUST] Decouple sending builder registrations to the BN from the trigger of proposer duty. Currently this line of code, a request to the BN (and subsequently to the MEV-boost/Commit-boost), blocks the execution of this line, which initiates the start of a duty. We should make the builder call async.
2. [RECOMMENDED] As the call of builder registrations will be async, most likely it will execute after we ask for proposal data. It would be nice to have it executed once on Charon start.
3. [OPTIONAL] Rework how often / when the builder registration call is made. We first need to check if all MEV relays are fine with less frequent builder registration calls:
   • Making the call every 8 or 16 epochs will probably be OK. @OisinKyne any opinion on that?
   • Do not make the call at the epoch start. Probably doing it around end of slot 0 will be better, when there is less load on the BN.