miner: add OpenTelemetry spans for block building path

[jrhea]

Instruments the block building path with OpenTelemetry tracing spans.

• added spans in forkchoiceUpdated -> buildPayload -> background payload loop -> generateWork iterations. Spans should look something like this:

jsonrpc.engine/forkchoiceUpdatedV3
|- rpc.runMethod
|  |- engine.forkchoiceUpdated
|     |- miner.buildPayload [payload.id, parent.hash, timestamp]
|        |- miner.generateWork [txs.count, gas.used, fees] (empty block)
|        |  |- miner.prepareWork
|        |  |- miner.FinalizeAndAssemble
|        |     |- consensus.beacon.FinalizeAndAssemble [block.number, txs.count, withdrawals.count]
|        |        |- consensus.beacon.Finalize
|        |        |- consensus.beacon.IntermediateRoot
|        |        |- consensus.beacon.NewBlock
|        |- miner.background [block.number, iterations.total, exit.reason, empty.delivered]
|           |- miner.buildIteration [iteration, update.accepted]
|           |  |- miner.generateWork [txs.count, gas.used, fees]
|           |     |- miner.prepareWork
|           |     |- miner.fillTransactions [pending.plain.count, pending.blob.count]
|           |     |  |- miner.commitTransactions.priority (if prio txs exist)
|           |     |  |  |- miner.commitTransactions
|           |     |  |     |- miner.commitTransaction (per tx)
|           |     |  |- miner.commitTransactions.normal (if normal txs exist)
|           |     |     |- miner.commitTransactions
|           |     |        |- miner.commitTransaction (per tx)
|           |     |- miner.FinalizeAndAssemble
|           |        |- consensus.beacon.FinalizeAndAssemble [block.number, txs.count, withdrawals.count]
|           |           |- consensus.beacon.Finalize
|           |           |- consensus.beacon.IntermediateRoot
|           |           |- consensus.beacon.NewBlock
|           |- miner.buildIteration [iteration, update.accepted]
|           |  |- ...
|           |- ...

• added simulated server spans in SimulatedBeacon.sealBlock so dev mode (geth --dev) produces traces that mirror production Engine API calls from a real consensus client.