Add a simple server to test against

[kiike]

I wanted to help solve #58 and learn more about the library, so I created a simple web server to test against. It provides a single method to add two integers as a building block for AGI. Since the servers runs locally, no internet access is needed, plus we can ensure the tests don't rely on an external service.
This PR does not depend on #62, but won't pass cleanly without it, due to using Async\Await in a function instead of Clue\React\Block.

Fixes #58.

[PaulRotmann]

Thanks for working on this! Adding a test server to eliminate external dependencies is definitely a good approach for solving #58. The concept of having a local server for testing is solid.

However, I can see that the CI tests are failing, likely due to PHP compatibility issues. The tests appear to be written for newer PHP versions but our current CI setup tests against PHP 7.1+. Could you make sure your code is compatible with PHP 7.1 so the tests pass?

Once the compatibility issues are resolved and the tests are green, we'd be happy to review this properly. The functionality itself looks useful, it just needs to work with our supported PHP versions.

Thanks for the contribution!