feat: add mailpit as module

[feslima]

What does this PR do?

Adds Testcontainers.Mailpit as a module to the project.

Why is it important?

Mailpit is an extremely useful tool for testing email related features and I think having it as a module to use inside the Testcontainers .NET C# ecosystem is worth the effort.

Related issues

Didn't find any.

How to test this PR

I've added simple a test that ensures the SMTP and Web servers are working accordingly by sending the email and querying the REST api for that specific email.

To run this test in your shell:

dotnet test "tests/Testcontainers.Mailpit.Tests/Testcontainers.Mailpit.Tests.csproj"

Follow-ups

If this PR is acceptable, I think there is still some tidying-up of the .sln files to keep consistency as I've seen done in other merged PRs.

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[HofmeisterAn]

Sorry for the late response. I will try to review the PR next week :v:.

[HofmeisterAn]

Relates to #1153.