host: type chain_id across the component seam via alloy-chains

[mfw78]

Problem. ChainProvider and CowApi take chain_id: u64, losing the type safety and display/known-chain conveniences a dedicated chain type provides.

Proposed. Adopt alloy_chains::Chain (or its ChainId alias where a plain id is genuinely wanted) across the seam traits and outward through the config/pool layer where it reduces conversions. Check whether alloy-chains is already in the dependency graph via alloy before adding it as a direct dependency.

Notes. Applies to both component/chain.rs and component/cow.rs. Raised in review feedback on the component-seam PR: #99 (comment) and #99 (comment)

[mfw78]

Merged in #110.