feat: add new engine_opSealPayload API

[bnoieh]

Description

This PR adds new engine_opSealPayload API, which can be enabled by flag --sequencer.combined-api.
If enabled, sequencer will call engine_opSealPayload instead of engine_getPayload/engine_newPayload/engine_forkchoiceUpdated to confirm unsafe payloads when producing block.

Rationale

Through load test, we found that it took about 200ms(10k tps case) to call/serve of engine apis. So we add this new API to reduce the latency to almost 0ms here to achieve higher TPS.

Example

n/a

Changes

Notable changes:

• use new flag --sequencer.combined-api to change to call new engine_opSealPayload API when producing unsafe block

# original flow
1. op-node call engine_forkchoiceUpdated
2. wait scheduled time
3. op-node call sequentially: engine_getPayload -> engine_newPayload -> engine_forkchoiceUpdated

# new flow when enable --sequencer.combined-api
1. op-node call engine_forkchoiceUpdated
2. wait scheduled time
3. op-node call concurrently: engine_opSealPayloadResponse & engine_getPayload

• only unsafe block building will use engine_opSealPayload