feat(op-reth): register eth_config RPC endpoint

[joshklop]

Wire EthConfigHandler into OpAddOns so the eth_config method (EIP-7910) is served on the eth namespace. Extend the generic bounds on the NodeAddOns and RethRpcAddOns impls to satisfy EthConfigHandler::new: ChainSpec must also implement Hardforks (OpHardforks only implies EthereumHardforks), and the primitives' header must implement HeaderMut so per-fork configs can be built by cloning and retiming the latest header.

Implement HeaderMut for the custom-node example's CustomHeader so the example keeps compiling.

Add an integration test in crates/node/tests/it/rpc.rs that launches an OpNode against BASE_MAINNET and calls eth_config via the jsonrpsee HTTP client, verifying the endpoint is wired up.

[wiz-0f98cca50a]

Wiz Scan Summary

Scanner	Findings
Vulnerabilities	-
Sensitive Data	-
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IaC Misconfigurations	-
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[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 0.5%. Comparing base (de1b557) to head (8ef1cd9).
⚠️ Report is 5 commits behind head on develop.

❗ There is a different number of reports uploaded between BASE (de1b557) and HEAD (8ef1cd9). Click for more details.

HEAD has 13 uploads less than BASE

Flag	BASE (de1b557)	HEAD (8ef1cd9)
cannon-go-tests-64	1	0
contracts-bedrock-tests	12	0

Additional details and impacted files

@@             Coverage Diff             @@
##           develop   #20158      +/-   ##
===========================================
- Coverage     75.8%     0.5%   -75.3%     
===========================================
  Files          183      486     +303     
  Lines        10536    61736   +51200     
===========================================
- Hits          7987      320    -7667     
- Misses        2405    61416   +59011     
+ Partials       144        0     -144     

Flag	Coverage Δ
cannon-go-tests-64	?
contracts-bedrock-tests	?
unit	0.5% <ø> (?)

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