op-service: add `eth.ChainConfig` interface, replace `*params.ChainConfig`

We don't want to leak the underlying `*params.ChainConfig` type of geth everywhere, since in most cases we only want to check if a fork is active, and don't need concrete-type access or config mutation.

Start in op-service by locating uses of *params.ChainConfig and reading how callers check fork activation or access configuration. Done means consumers can depend on the proposed eth.ChainConfig interface without leaking the geth concrete type, while preserving the existing fork checks.

Written by the indexing model from the issue text.