Add Testcontainers for .NET to enable database tests in CI

[HofmeisterAn]

Hi 👋, thank you for maintaining Respawn. It's an awesome library, and I will definitely use it more often in the future.

While working on a small example demonstrating Testcontainers for .NET together with Respawn, I noticed that some of your tests are disabled in CI. For many of the database tests, we have working modules that could likely help integrate and enable these tests in CI. If you'd like, I can take a look at some of them and make the necessary changes to incorporate Testcontainers.