Add Temporal codec-server endpoints

[raszi]

The work in #85 and my follow-up PR made it possible to use the data converter that the client uses directly.

This opens up the possibility to ship ring-compatible functions that could be mounted to web applications to encode and decode messages.

What I imagined is two higher-order functions that receive an optional data converter or fall back to the default if none is provided, and return a ring-compatible function that could be mounted in web apps.

See https://docs.temporal.io/production-deployment/data-encryption#sample-requestresponse

[raszi]

Since the data converter could potentially depend on the namespace, it might need to receive a function as its argument that receives either the whole request or just the namespace and returns the data converter to be used.