payload,rpc,txpool: expose pending build extension points

[joshklop]

Description

Expose generic reth extension points needed by sequencer-defined pending base fees:

• expose the payload builder state-root receiver
• allow callers to supply a pending base fee to transaction-pool maintenance
• allow RPC implementations to supply pending base fees for fee suggestions, pending fee history, transaction filling, pending EVM environments, and multi-block simulation
• preserve canonical child base fees for historical fee-history entries

This remains chain-neutral: callers opt into the hooks, while existing callers retain the standard Ethereum behavior.

Validation

• cargo +nightly fmt --all -- --check
• cargo test -p reth-transaction-pool test_maintain_transaction_pool_with_custom_base_fee
• cargo check -p reth-rpc-eth-api --all-features
• full CircleCI reth workflow passed on the initial branch revision

Dependencies

None. The downstream Optimism SDF draft will pin this branch revision while this PR is under review.

Review status

Draft: final review and CI follow-up are still in progress.