Gradually introduce SubclassOptInRequired into our codebase

[qwwdfsad]

https://kotlinlang.org/api/latest/jvm/stdlib/kotlin/-subclass-opt-in-required/

Historically, we have multiple interfaces that are not designed for external implementation, , but are not sealed for various reasons (including historical ones):

• Job should not be extended externally
• Deferred and CompletableDeferred should not be extended externally
• Channel probably should not be extended externally
  • It is not mentioned in the documentation, and it's impossible to fulfil the select's contract with a hand-rolled implementation
  • Yet it is possible to implement it with a delegation, potentially closing the possibility of adding new abstract members to the interface

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• Flow, SharedFlow, and StateFlow are unstable/experimental for inheritance
• CloseableCoroutineDispatcher is unstable for inheritance
• CancellableContinuation is an interface not ready to be extended

It would be nice to adopt the SubclassOptInRequired (available from 1.9.0) in the library to provide explicit programmatic warnings/errors and to prevent issues like #3729

For "prohibited to implement" interfaces we can introduce the new annotation with a warning (or maybe even an error) and, in the next major release, make the annotation internal itself, so it would be much harder to opt-in

[dkhalanskyjb]

Yet it is possible to implement [a Channel] with a delegation

Yep, even a naive search yields a couple of places: https://grep.app/search?q=by%20channel&filter[lang][0]=Kotlin