Move `GULLogger` to `os_log` for iOS 10+

[ryanwilson]

Currently we use asl_log for all logging within GULLogger, but it's deprecated. os_log was introduced in iOS 10 as a faster and more robust logging solution.

We should move to os_log for all iOS versions above iOS 10, and use the old asl_log for iOS 8 and 9 until we drop support for those versions.

[morganchen12]

Unassigning bstpierr since he no longer works on Firebase.

[paulb777]

Alternative would be to directly use os_log from the product SDKs and deprecate FIRLogger and GULLogger.

First step will be to review https://developer.apple.com/documentation/os/logging and come up with a design

[mortenbekditlevsen]

Just providing another alternative - in case running on non-Darwin platforms were a goal - and for the frameworks that are already in Swift. :-)

https://github.com/apple/swift-log

Which could then on Darwin platforms be used to log to os_log through:

https://github.com/chrisaljoudi/swift-log-oslog And on other platforms, another logging client could be injected.

Just a thought 😊

[paulb777]

Deprioritizing for Firebase 10 in favor of thinking about a logging strategy that is Swift first