Add interface to SshCommand

[MartinHvolgaardHansen]

This update adds an interface to SshCommand to enable easy mocking in dependent tests.

A quick overview of the changes:

1. Add the ISshCommand interface.
2. Move breaking changes to implementations to the V2 namespace to ensure backwards compatibility.
3. Change existing implementations to be thin wrappers around changed implementations.
4. Update and run tests associated with the changes (except the Docker integration tests).
5. Update examples in doc files to reflect these changes.

There are alternatives to achieve testability, but for SSH.NET to be truly testable in future projects, this interface is needed.

Did I miss anything? Let me know, and I'll make sure to amend it.

Thanks for reviewing and your time, and a big thanks to the authors and contributors.