256 activation epoch check

[valefar-on-discord]

In order for a validator to exit, withdraw or consolidate, the validator needs to have been active for at least 256 epochs. If not, any attempt to perform those actions will fail. Currently the tool does not check for this but should.

The API provided validator.activation_epoch which can be used against the current epoch of the selected chain to verify this check.

https://github.com/ethereum/consensus-specs/blob/master/specs/electra/beacon-chain.md#voluntary-exits
https://github.com/ethereum/consensus-specs/blob/master/specs/electra/beacon-chain.md#new-process_withdrawal_request
https://github.com/ethereum/consensus-specs/blob/master/specs/electra/beacon-chain.md#new-process_consolidation_request

    # Verify the source has been active long enough
    if current_epoch < source_validator.activation_epoch + SHARD_COMMITTEE_PERIOD:
        return