Improve logging and support diagnostic tooling

[kkostov]

Presently, debug output from the FlutterSDK is somewhat difficult to use.

• Debug messages from the SwiftSDK are not visible in the Flutter debug console unless the app has been started directly from Xcode. Furthermore, such messages are not usually detectable by Flutter logging frameworks.

• Debug messages from the KotlinSDK are visible, but not very easy to control as the output can't be tagged / redirected to a logging framework.

To improve this, we can:

• (Dart) Implement a custom abstract LogHandler class to be registered as a callback function for both SwiftSDK and KotlinSDK. The log handler drops all messages and does not print anything by default.
• (Swift/Kotlin) The native integration for both platforms will capture all log messages and forward them to the Flutter VM.
• (Dart) Adapt TelemetryManagerConfiguration to accept a logHandler callback as an option. Client apps can implement the longHandler and redirect messages to their preferred logging framework.
• (Dart) Mark the existing debug option as deprecated. As a fallback, we can provide a default print() implementation when debug is set to true, this will cause a warning so developers will be prompted to migrate.

Any thoughts or comments are welcome @winsmith @Jeehut

[Jeehut]

@kkostov I don't quite understand the limitations of Flutter/Dart here. But in general we should a) use what most developers want by default on the related platform (being OSLog in the Swift SDK), but b) support a way that users can integrate with their existing custom loggers where needed.

I'm not sure if you mentioned me because you need a change in the Swift SDK, but as you mentioned we already have the logHandler you can override by default in the Flutter SDK to whatever is expected on that platform. Then, you could additionally provide a custom override, too if needed.

[kkostov]

@Jeehut Thanks for your insight! No changes are needed in the SwiftSDK, I just wanted to touch base in case there have been discussions in this context.