Port Ice Swift src to use async/await

[externl]

This PR removes PromiseKit and replaces the Promise based API with async/await. We make use of the Swift/ObjC async interop support as well as the Swift's withCheckedContinuation helper function.

• https://developer.apple.com/documentation/swift/calling-objective-c-apis-asynchronously
• https://developer.apple.com/documentation/swift/withcheckedcontinuation(function:_:)

CI is failing as I have not disabled or ported the tests yet (I have some done locally). Just opening this draft PR for feedback. There's also still some cleanup to be done.

Things of note:

• We no longer need the C++ dispatch queues. The C++ threads hand off to the global executor roughly when we switch from Objc++ to Swift. As a consequence of this thread switch there's no way to provide back pressure by hogging the C++ thread.
• The optional test takes a long time to compile. Not sure what we can do about it.

Future cleanup and improvements:

• Port a few tests which are currently commented out
• Remove sync API entirely (I'm a little concerned about compile time)
• Remove sent callback
• Implemented MaxDispatches per OA

[bernardnormier]

Looks good.

Larger question: what's your plan in terms of sync API for proxies and servants?

• keep sync + async APIs like in other language mappings
• provide only an async API like in IceRPC, and the current sync + async APIs is a temporary step?

[bernardnormier]

Good start!