Accept any EventLoops in `.delegate(on:)`, not just ones from AHC's own ELG

The badly named but highly useful .delegate(on:) specifies two things:

1. Which EventLoop the returned futures are bound to
2. Which EventLoop the delegate callouts happen on.

Crucially it does not specify on what EventLoop the underlying Channels are running on. Yes, as an optimisation if possible & sensible (i.e. the connection pool doesn't already have a valid connection) we try to create new Channels on the same EventLoop but that's neither guaranteed nor required.

Expected behaviour

If I pass a random EventLoop to AHC's eventLoop: .delegate(on: myRandomEL) then I would expect this to work, even if AHC itself is using an EventLoopGroup that doesn't contain my EventLoop for its Channels.

Actual behaviour

AHC crashes and tells me that I can only use EventLoops from AHC's EventLoopGroup at runtime. That's bad an unnecessary.

.delegate(on:) のエントリーポイントから開始し、渡された EventLoop を AHC 独自の EventLoopGroup に制限しているランタイム検証を追跡します。別のグループの EventLoop を使ったテストカバレッジを追加し、そのうえで、基盤となる Channels が引き続き AHC によって管理されながら、クラッシュせずに委譲が機能することを確認します。