Add InMemory Support to enable testing

[radderz]

Would it be possible to get these same generalized exceptions using the "InMemory" efcore option?

[Giorgi]

@radderz Does the in-memory provider throw exception for validations that fail (such as length, unique, and so on). If the does it might be possible to get the same exceptions but I think it doesn't. It definitely doesn't validate requiredness but it will be added in EF Core 6: dotnet/efcore#10613

[radderz]

I see that the in memory database isn't a relation database which is not what I expected, I can probably use an in memory sqlite instance to do the same and you already have an sqlite version.

[Giorgi]

Yes, in-memory SQLite is probably a better choice for testing.