Add UnknownServiceException

[Goooler]

I noticed Kotr is migrating to kotlinx-io in ktorio/ktor#4032, and I sent a PR ktorio/ktor#3725 which is depending on UnknownServiceException, it would be awesome to port this exception for all platforms to get rid of custom implementations in Ktor.

[fzhinkin]

Hey @Goooler!

Thanks for opening the PR. Unfortunately, I don't think that this exception is a good fit for the kotlinx-io right now for a few reasons:

• currently, kotlinx-io provides no networking-related functionality;
• having only UnknownServiceException and not having other exceptions covering various networking and URN/URL-related problems feels a bit off.

So for now, I'd suggest leaving the exception in the Ktor codebase.

However, in the long run, it would make sense to supply a separate package stuffed with networking-related exceptions to serve as a common types for the ecosystem.

[Goooler]

Makes sense!