UnwrapAsAlb should be a transformer to match the pattern of other AWS transformations

[jbohanon]

When wrapAsApiGateway and unwrapAsApiGateway were implemented, they were implemented as custom transformers which were defined in envoy-gloo-ee to protect them as enterprise features. After moving unwrapAsApiGateway into OSS, and looking ahead toward other potential request/response transformer implementations, it makes sense to extract the logic for unwrapAsAlb out into a custom transformer as well. This will be more uniform and will serve to simplify the internal API