feat: Change return type on WithSelfMock

[BenjaminMichaelis]

This is a proposal as well as showing it.

This is all based on my assumption of how AutoMocker works :)
I think returning the mocked service here makes more sense than the object of the implementation.

1. Having the actual instance only helps if I am about to go use the concrete type after (saving me one line of code at most).
   ex:

var implementedObject = Mocker.WithSelfMock<IStorage, RealStorage>();
foo.TryRenameAsync(implementedObject, newName))

But could be achieved by doing this in the new format:

var serviceMock = Mocker.WithSelfMock<IStorage, RealStorage>();
foo.TryRenameAsync(serviceMock.Object, newName))

2. With the mock service, I can then so do a setup that I couldn't do immediately with the implemented object.
   before:

Mocker.WithSelfMock<IStorage, RealStorage>();
Mock<IStorage> mock = Mocker.GetMock<IStorage>();
mock.Setup(mock => mock.DoThing(name)).ReturnsAsync("Did Something");
foo.TryRenameAsync(mock.Object, newName))

can now be:

Mock<IStorage> mock = Mocker.WithSelfMock<IStorage, RealStorage>();
mock.Setup(mock => mock.DoThing(name)).ReturnsAsync("Did Something");
foo.TryRenameAsync(mock.Object, newName))

3. The user doesn't loose out from the serviceMock being able to represent the concrete class, and can still call it if call base is passed in.

Mock<IStorage> mock = Mocker.WithSelfMock<IStorage, RealStorage>(callBase: true);
mock.Setup(mock => mock.DoThing(name)).ReturnsAsync("Did Something");
foo.TryRenameAsync(mock.Object, newName))