feat: add sequencer/rpc mode for flashblock preconfirmations

[fridrik01]

This PR adds flashblock support to bera-reth and introduces two modes:

• Sequencer mode: the node produces flashblocks at ~200ms intervals during block building and broadcasts signed partial blocks over WebSocket.
• RPC mode: the node subscribes to a sequencers websocket stream and serves in-progress flashblock data through the standard Eth JSON-RPC layer, enabling transaction preconfirmations.

Sequencer mode (--sequencer-enabled)

• --flashblock-signing-key <path> Path to BLS secret key file (required)
• --flashblock-ws-addr <addr> WebSocket bind address (default: 0.0.0.0:8548)
• --flashblock-interval-ms <ms> Emission interval (default: 200)

The payload builder runs a loop that (as in line with preconf notion doc):

1. Checks stop conditions and exits when:

• getPayload is called (block finalization requested)
• Deadline exceeded (--builder.deadline)
• Gas limit reached (block full)

2. Emits flashblocks every 200ms and publishes current progress regardless of transaction activity (empty flashblocks serve as heartbeats)
3. Pulls txs from mempool, executes them, tracks gas and fees
4. Emits final flashblock on loop exit with is_last: true so RPC nodes know no more updates are coming

Each flashblock contains:

• payload_id / index Identifies position in the block building sequence
• base Block header fields (only in index 0)
• diff Incremental transactions, receipts, and state changes
• signature BLS signature over the diff for authentication
• is_last Signals final flashblock for this payload

Also, a single publisher broadcasts to all connected WebSocket subscribers.

RPC consumer mode (--flashblocks-url )

RPC nodes connect to a sequencer's WebSocket endpoint and subscribe to flashblocks. Incoming flashblocks are processed and used to override the Eth JSON-RPC layer so that latest and pending queries reflect sub-block state. Blocks, transaction counts, receipts, state, gas price, and call execution all serve in-progress flashblock data without the node needing to run as a sequencer.

Test plan

See test plan in berachain/beacon-kit#3005

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✨ Finishing Touches

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• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch sequencer-dev

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