FR: Expose Custom Networks

The alchemy network naming convention is very abnormal; it would be extremely helpful to expose [CustomNetworks](https://github.com/alchemyplatform/alchemy-sdk-js/blob/master/src/util/const.ts#L165C14-L165C28) via the SDK to allow for easier lookups. This can easily be paired with tools such as CAIPs that rely on chainID rather then some custom network name
