Source Generators: should user generators require an attribute for discovery?

[chsienki]

Currently generators are required to both implement ISouceGenerator and have the [GeneratorAttribute].

We only use the attribute so we could reuse the same discovery mechanism as analyzers. Should we remove this restriction?

[chsienki]

As part of VB generators in #49139 we've added a language param to the attribute, so it is now required.