op-node: block signing and publishing improvements

In summary:

- Signing block happens inefficiently, at the less-than-ideal time, and can use documentation.
- Publishing blocks happens with bottleneck and at the less-than-ideal time.
- Committing blocks and sequencer rotation is prone to async-gossip race conditions.

Signing efficiency is fixed with https://github.com/ethereum-optimism/optimism/pull/14478
combined with https://github.com/ethereum-optimism/infra/pull/203

And the V2 API fixes the typing, so alternative remote-signer services can be implemented, on top of a sane API format.

Blocks are currently "async-gossiped". This becomes obsolete with flash-blocks, and never won us that much time.
It did however add fragility: when a block is committed to conductor before it is published,
then there is a continuation issue, where other nodes cannot continue without receiving the missing block.

The conductor does post the missing block with `admin_postUnsafePayload`,
but that doesn't retry or lock, which means the conductor can still fail to handover sequencer control
when the block has not been processed yet, even after success-response.

On top of that, the conductor does not get the block-signature, nor posts the signature back to the node.
This makes sequencer-rotation extra difficult, since a block from the conductor cannot be published to verifiers.
So when the sequencer rotation fails, there can be a verifier hiccup where the block is temporarily missing, since the conductor does not post it everywhere, and block cannot be gossiped out without signature.

We should make the sequencer:

- Handle block signing/committing/posting as following:
- Sign the block before committing to it.
- Post the signature to the conductor, and include the signature.
- Receive the block from the conductor with a commit-API, with signature, *synchronously*,
to ensure the block is there before rotating sequencer responsibility to the node.
- Receive the block from the conductor with a publish-API, with signature, to republish.
- Start using the V2 API:
- Roll out the op-signer change with the V2 block-signing method
- Add v2-block-signing flag to op-node, see [[issue 14499](https://github.com/ethereum-optimism/optimism/issues/14499)](https://github.com/ethereum-optimism/optimism/issues/14499)
- Document the V2 API format

V2 API usage is low-lift and can happen today.
The signing/committing/posting improvement may not be worth it,
and instead lean on the sequencing work to get completed.