[WIP/QRCoder2] Consider Removing Nested Types from Public API Signatures

[csturm83]

MS design guidelines on the use of Nested Types suggests that they should be used sparingly and avoided in public API (amongst other recommendations).

PayloadGenerator comes to mind as well as some public enums in QRCodeGenerator. The nesting of types could be removed where unnecessary.

[codebude]

I don't think there's anything to add here. Where it makes sense, we can remove/unnest the nested types.

[codebude]

I added this as "decision" to the descision post of the meta issue. Thus I'll close this issue here, so that we can mark it as completed/decided in the "things to discuss" list here.