Add example package using shared types in client and server

[simonjbeaumont]

Motivation

We get asked from time to time how to structure a package to share types from an OpenAPI document between multiple targets, e.g. a client and a server. This allows adopters to write extensions or other functionality that uses the common types once, and use them from both downstream modules.

Modifications

Add an example package that combines the existing Hello World client and server but factors the types generation into a separate module.

Result

Another example package.

Test Plan

• Tested locally, which produces the same result as in the example README.
• CI.