[BREAKING] In order to support convention registration, generated class and namespace are no longer customizable

[kzu]

Since the source generator needs to be able to analyze code invoking the AddService overloads, these can no longer be emitted by another source generator, since those files do not participate in subsequent analyzer/generator runs.

So we moved those files to be contentFiles in the package (with Compile action) and therefore we can no longer offer the previous properties to customize the namespace and class name.

This was likely seldom used anyway, and doesn't add much value anyway. We should rename the extension class just in case, to make it less probable to have a naming conflict.