feat(Flush): add new flush method for Android/iOS

[danielgomezrico]

If you are in debug mode, been able to flush events, It's pretty helpful.

Currently, this is not allowed

Docs

• iOS: https://segment.com/docs/connections/sources/catalog/libraries/mobile/ios/#flushing
• Android: https://segment.com/docs/connections/sources/catalog/libraries/mobile/kotlin-android/#flush