Add structured logging as an alternative to stdout printing

[davidrothera]

This fixes #131 and adds a oslog handler for logging as an option.

Rather than change all logging to use this new endpoint by default I left it as an option for the client to decide as perhaps people could be relying on stdout printing.

The new variable is gated around the platforms which support this new structured logging API (iOS 14 and similar).

[davidrothera]

One option which I did consider was a more thorough change to logging where we could pass through the subsystem (in oslog parlance) of where the log comes from but I didn't implement this as the library isn't large enough that the call site could be one of 100 subsystems.

[Jeehut]

Closing this in favor of https://github.com/TelemetryDeck/SwiftSDK/pull/195. Thanks again @davidrothera!