feat: rework execution requests and admit joining validators early to the p2p network

[matthias-wright]

This updates the execution request handling in the finalizer to uphold the following invariants:

• A validator will join the committee VALIDATOR_NUM_WARM_UP_EPOCHS epochs after submitting a valid deposit request. The phase after submitting the deposit request, and before joining the committee is called the onboarding phase.
• If a withdrawal request is submitted in epoch n, then the validator will be removed from the committee at the end of epoch n. The withdrawal will be processed in epoch n + VALIDATOR_WITHDRAWAL_NUM_EPOCHS.
• A validator can only submit one withdrawal request at a time. If another withdrawal request is submitted, while a withdrawal request is pending, then the second withdrawal request will be ignored.
• If a withdrawal request is submitted while a validator is in the onboarding phase, then the onboarding phase is aborted, and the withdrawal request will be processed VALIDATOR_WITHDRAWAL_NUM_EPOCHS epochs later.
• No partial withdrawals. If the validator balance is balance, and a withdrawal request with amount amount < balance is submitted, then the withdrawal request will be processed for the amount of balance.
• A validator can only have a balance of VALIDATOR_MINIMUM_STAKE. If a deposit request with amount is submitted, where amount != VALIDATOR_MINIMUM_STAKE, then the deposit request will be skipped, and a withdrawal request will be initiated immediately.
• No top up deposits. If a validator already has a balance of VALIDATOR_MINIMUM_STAKE, then it cannot submit another deposit request with amount VALIDATOR_MINIMUM_STAKE.

Additionally, this adds joining validators to the p2p network early. When a validator stakes in epoch n, they will join the committee in epoch n + VALIDATOR_NUM_WARM_UP_EPOCHS. However, they will now be added to the p2p network in epoch n + 1. This allows them to sync blocks and catch up their state, without actively participating in consensus.