Add async await support in place of completion handlers

[swhitty]

Is your feature request related to a problem? Please describe.
The current Swift API is lacking support for async / await. While developers can naively wrap the existing methods with continuations ignoring cancellation, native support for async / await with support for cancellation would be preferred.

Describe the solution you'd like
Key APIs that currently include completion handlers also offer an async await extension

public extension LDClient {

  static func start(config: LDConfig, startWaitSeconds: TimeInterval) async throws { .. }

  func setOnline(_ goOnline: Bool) async throws { .. }
  
  func identify(context: LDContext) async throws { .. }
  
}

Describe alternatives you've considered
I currently write these extension myself but there are a few issues due to limitations within the public API:

• start() can call its completion handler multiple times if timeout expires before it becomes online.
• start(), setOnline() & identifiy() have no API to cancel causing completion handler to receive failure. e.g. URLSessionTask.cancel()

The workarounds for these issues are non trivial.

[galenqq]

Would love to see this!

[tanderson-ld]

Thank you for the feedback! We will take it into consideration.