feat(gasprice): Add gas price controller (default mode)

[Copilot]

Implements XLayer gas price controller with default mode that dynamically adjusts L2 gas prices based on network congestion while maintaining a configured minimum floor.

Architecture

Gas Price Controller (crates/xlayer/gasprice/)

• DefaultGasPricer: Returns fixed configured price, no external data dependencies
• XLayerScheduler: Periodic updates based on L2 congestion metrics (pending txs, block fullness)
• GasPriceCache: Thread-safe price storage with raw/adjusted values

Node Integration (crates/optimism/node/)

• Extracted XLayer initialization to node_xlayer.rs
• Wires scheduler → EthAPI oracle + transaction validator
• Spawns xlayer-gas-price-scheduler background task

RPC Layer (crates/optimism/rpc/, crates/rpc/rpc/)

• Implemented XLayerFees trait on OpEthApi for pricer management
• Separated XLayer-specific logic to mod_xlayer.rs
• Added eth_minGasPrice RPC method

Congestion Logic

// L2 gas price = max(base_fee + tip_cap, default, raw_gp)
let l2_price = max(base_fee + tip_cap, default_price);
let result = max(l2_price, raw_gp);

// If congested (pending >= threshold && block not empty), use result directly
// Otherwise, average with raw_gp to gradually decrease
if !is_congested {
    result = (raw_gp + result) / 2;
}

Configuration

--xlayer.gasprice.type default \
--xlayer.gasprice.default 100000000 \
--xlayer.gasprice.update-period 3s \
--xlayer.gasprice.congestion-threshold 100

Future modes (follower, fixed) will calculate raw_gp from L1 prices; default mode uses configured value.

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