`structs: dependency-only: opaque` should apply to ObjC interfaces

Say we have an API that requires an `NSString` as an argument.
Say you only want to call that API, not do all the fiddly bits you can do with `NSString`s so you set `exlcude-all-by-default` and only include the interface class you want to use.

If you happen to run `ffigen`, it will see the API you want to use needs an `NSString`, then it will have a look at all its APIs and will go ahead and generate code for all of those. While this is functional, it creates a lot of code (file sizes measured in megabytes). I assume this code does get tree-shaken during compilation, but it's still something that will impact compilation speed, git repo size and all the implications that has over time, package size when pushed to pub.dev (and being downloaded by the package users), etc.

I've also tried `structs: dependency-only: opaque` but that doesn't seem to do anything.

While the approach to generate all the dependent code is a valid one and it does work, I wonder if we could make `exclude-all-by-default` apply for this as well. Instead, I'd prefer having to specify the methods I need (besides the constructor) - I am excluding everything after all. Alternatively, it could be done so that by just listing the class/interface in the include list would generate all methods (related: https://github.com/dart-lang/native/issues/251) but would not go on transitively for each argument - those would be opaque instead.

Example:
* ffi config: https://github.com/getsentry/sentry-dart/pull/1622/files#diff-6101fa2fb8fa86e958fe3a953e7e9f5390f14cf1f4366ec5d4e39a3eb6f7857d
* generated file: https://raw.githubusercontent.com/getsentry/sentry-dart/8367da2a088bb6e5ece64d09d53f074233b26ba2/flutter/lib/src/sentry_cocoa.dart