Chain enum is missing ZKS chain

[ben-iron]

When generating a client for a strongly typed language such as Rust from this OpenAPI spec, it will spectacularly fail because the list stablecoins endpoint returns an entry for the ZKS chain. Since Rust converts the OpenAPI enum into a proper enum and not something stringly typed, parsing the response will fail.

I'm happy to provide a PR if there is a policy to merge external PRs.