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.

从 .delegate(on:) 入口开始，跟踪将传入的 EventLoop 限制为 AHC 自有 EventLoopGroup 的运行时验证。使用来自独立 group 的 EventLoop 添加覆盖，然后验证委托能够在不发生崩溃的情况下工作，同时底层 Channels 仍由 AHC 管理。