feat(flashblocks): Flashblocks engine sidecar

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Towards: https://github.com/paradigmxyz/reth/issues/18255, see bottom for follow up needed.

feat(op-reth): integrate flashblocks with consensus engine

Summary

This PR completes the integration of flashblocks with the consensus engine by:

• Adding structured conversion infrastructure from flashblock sequences to execution payloads
• Automatically spawning the consensus client when flashblocks are enabled
• Improving error handling and observability with detailed logging

Integration

crates/optimism/rpc/src/eth/mod.rs

• Added automatic spawning of FlashBlockConsensusClient when flashblocks are enabled
• Wired up beacon_engine_handle to enable payload submission

crates/node/builder/src/rpc.rs

• Exposed beacon_engine_handle in EthApiCtx for RPC builders to access

examples/custom-node/src/engine.rs

• Added example implementation of From<&FlashBlockCompleteSequence> for custom execution data types
• Shows how to derive custom extensions from flashblock sequences

Dependencies

crates/optimism/flashblocks/Cargo.toml

• Added: op-alloy-rpc-types-engine (required for OpExecutionData)
• Removed: ringbuffer (no longer needed with simplified FCU logic)

Testing

• All existing tests pass
• New unit tests for conversion functions
• Clippy clean with all features enabled

Follow up needed

• ~~handle optional state root calculations scenario~~ Done
• ~~handle Jovian upgrade situation where blob_gas_used is not 0, need to update rollup-boost, op-rbuilder, and this repo for consistent types.~~ Done
• ~~ideally we move all the flashblocks relevant data structures into op-alloy-rpc-types-engine~~ Done
• add functionality to work with different forks