feat(gas): derive cold storage cost from gas params

[daixihegu]

Summary

Make GasParams::cold_storage_cost() behave as a helper by deriving the cold storage cost from cold_storage_additional_cost() plus warm_storage_read_cost().

This keeps the default Berlin value equal to COLD_SLOAD_COST, while also making custom gas parameter overrides compose correctly when either of the underlying values changes.

Closes #3342.

Testing

• cargo test -p revm-context-interface test_cold_storage_cost
• cargo fmt --all --check
• cargo test -p revm-context-interface
• cargo clippy -p revm-context-interface --all-targets --all-features -- -D warnings
• typos crates/context/interface/src/cfg/gas_params.rs
• git diff --check