Add requirement that channel is accessible in ServerChildChannel.Value

[adam-fowler]

This adds an additional requirement on the Value passed around by a ServerChildChannel that the Channel that spawned the child channel is accessible from the Value.

This PR doesn't add any additional functionality. It is here to prepare for the situation where we need the EventLoop for the Task executor.

[Joannis]

Isn't it better if we add var eventLoop: EventLoop here? This assumes we're using a NIO.Channel, whereas we could have other implementations such as Lambda that aren't connected to a socket at all.

I would even consider removing this requirement entirely, and replacing it with something like this:

func addTask<Failure: Error>(
  _ operation: @escaping () async throws -> Void,
  to taskGroup: inout ThrowingDiscardingTaskGroup<Failure>
)

[adam-fowler]

The ServerChildChannel assumes we are using a NIO.Channel anyway. It is there to setup a NIO child channel. This code is never run on lambda.

I'm not exactly sure how you would use the addTask function