Strip source generator attributes using [Conditional]

[Sergio0694]

Currently, all attributes used for the source generator feature are preserved in the compiled binaries, which just results in unnecessary binary size given these attributes are only ever used to guide the source generators at build time. We can strip them from the final assemblies by leveraging the [Conditional] attribute and using a dummy constant that final users will not have. Additionally, we might also set a custom one that we can then document, to still offer advanced users the ability to opt-in into preserving them, if needed.