Proof of Concept: Containerized Integration Tests

[lugehorsam]

This is a POC on how an SDK can use an extended Nakama image loaded with rpc functions to run integration tests against.

See https://github.com/heroiclabs/nakama-clientsdk-test for how this image is built and the associated rpc functions. The image it builds is currently on Docker Hub: https://hub.docker.com/repository/docker/heroiclabs/nakama-clientsdk-test

For test social provider keys, I propose an optional bind mount from the host machine and corresponding optional tests that are triggered in each SDK if such keys exist.

Remaining Steps:

(1) Make client-sdk test image repository public (before merging!)
(2) Versioning for test image on Docker Hub and make it public also (before merging!)
(3) Integrate with Github Actions
(4) Finalize approach for test keys

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