do not require an interface to leverage code-first

As a developer, I want to be able to authot a `partial` class with `partial` methods, which the source-generator picks up to generate typed accessors to resources, to not be forced to use an interface when I don´t really need it.

Inspired by the way `Microsoft.Extensions.Logging` is generating code, see:
https://learn.microsoft.com/en-us/dotnet/core/extensions/logger-message-generator
