Add initial implementation of a Server example (#1222)

We are adding this example service in the aws-smithy-http-server folder to showcase the SSDK and to be able soon to run benchmarks against this example.

This change includes a new model for the Pokémon Service and all the necessary infrastructure to code generate the client and server SKDs and make them available to the runtime implementation.

A basic README has also been added with instructions on how to build, run and test the service.

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