`op-node`: Unsafe Block RPC Endpoint

[tynes]

It is useful for some applications to have reliable access to the signed unsafe blocks. An RPC method could be added to the op-node to fetch this data. This RPC method would only be enabled if the node is running in sequencer mode

Some open questions on what this could look like:

• Fetch by hash/number, similar to how fetching a block works
• Returns both the signature and the block?
  • The block is raw RLP encoded?

Ideally the unsafe blocks can be easily accessed via the p2p network, but its currently unreliable. https://github.com/ethereum-optimism/optimism/pull/11781 is an attempt to improve the p2p network, but it isn't ready for merge yet.

[ncitron]

• I think being able to fetch by number as well as the latest is most useful. Block hash is a nice add on but isn't too important in my opinion as for an honest sequencer, there should be a one to one mapping from hash to block already.
• Currently the unsafe heads are gossiped as signed ssz encoded ExecutionPayload objects, so I would definitely keep it as the payload format so that it is easiest to verify the signature. I think many consumers would prefer it be json encoded, but ssz encoding works as well. Maybe some inspiration from the Ethereum Beacon api can be taken, which supports both ssz and json responses.
• Why should this method only be supported in sequencer mode? If I run an op-node instance myself it should have access to this data assuming it has a good gossip connection, and someone may want to grab them.