NIO's (Async)Channel should have a close API that doesn't throw if the Channel is already closed

[weissi]

In many -- likely the vast majority of -- cases, closing a(n) (Async)Channel may raise ChannelError.alreadyClosed. That's because the Channel can also be closed from the network side, so it's gonna be racy.

NIO should provide smth like

extension Channel {
   func close(ignoreAlreadyClosed: Bool) async throws {
       do {
            try await self.close()
       } catch let error as ChannelError, error == .alreadyClosed, ignoreAlreadyClosed {
           () // ignore
       } catch {
           throw error
       }
    }
}

or a similar API. And we should steer users towards using that by default.

[FranzBusch]

This already exist on the NIOAsyncChannel in the form of executeThenClose which ignores .alreadyClosed errors.

[weissi]

Cool, then we should just document that and also add it to Channel. Once we settle on an API, this can become a #good-first-issue