E2E testing infrastructure

[habbes]

While we do have tests to verify that the generate code looks as expected and compiles without errors, we don't have tests to verify that the code we generate runs as expected. We achieve that using manual testing, which is tedious and not enforced. We should consider setting up automated E2E testing to automate catching issues that we can currently only detect via manual testing.

For example, an E2E test could run the CLI to generate client code based on some schema and settings, add a predefined Program.cs file that calls the generated client code against some test OData service, and verify that the code ran as expected by inspecting the program output.