do not require an interface to leverage code-first

[earloc]

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