Add channel initializer closure to NIOAsyncTestingChannel.init

[hamzahrmalik]

Since nio 2.78, adding handlers to a pipeline requires the handlers to be sendable.

That makes the NIOAsyncTestingChannel.init(handlers:loop:) function cumbersome, because you cannot create handlers and then call the function (unless your handlers are Sendable) even if you never use the handlers elsewhere.

This PR adds a new initializer which takes a closure. The closure is run on-loop before the channel is registered. This means we can do:

let channel = try await NIOAsyncTestingChannel {
    let handler = MyUnsendableHandler()
    try $0.pipeline.syncOperations.addHandler(handler)
}

[Lukasa]

Nice one, thanks @hamzahrmalik!