WIP: capability to generate non Opt* struct for parameters with default value

[Liooo]

@tdakkota @ernado @shadowspore

https://github.com/ogen-go/ogen/issues/966

I saw some reactions on the issue and I've got some time now so diving into this. This PR includes only a minimal draft changes to discuss the implementation strategy, once the direction is consolidated I'm happy to do the rest of the work.

Just want a quick feedback at this point about if it's looking good or there's any other better ways. Honestly, haven't given much thought nor read the whole codebase yet so there could be some limitation in this strategy. Naming of the variables and types are also draft, will pick better ones if there's any along the way.

[ernado]

@tdakkota PTAL We need to decide how we going to implement this feature