Show how to change the provider when using WebApplicationFactory

[AndriySvyryd]

When using WebApplicationFactory to test the app changing the provider is currently not straightforward. See https://github.com/davidfowl/TodoApi/blob/aa5761d4ad3c516eae27d8a7fa41d8b97c812d63/TodoApi.Tests/DbContextExtensions.cs and https://github.com/davidfowl/TodoApi/blob/aa5761d4ad3c516eae27d8a7fa41d8b97c812d63/TodoApi.Tests/TodoApplication.cs#L39-L46

We should have specific guidance on how this can be done without bringing new dependencies to the app.

This can be simplified by adding AddDbContextOptions to EF