feat: add option to activate native integration with appsflyer

[johnsouza-loftbr]

Objective: add a new parameter on SegmentConfig to allow integrating Segment with Appsflyer.

Segment.config(
  options: SegmentConfig(
    writeKey: writeKey,
    trackApplicationLifecycleEvents: true,
    appsflyerIntegrationEnabled: true,
    amplitudeIntegrationEnabled: false,
    debug: kDebugMode,
  ),
);

Visual guide for Reviewers:

• 🚲 Bikeshedding.
• ⭐️ great increment!
• 🌟 awesome increment!
• ⛔️ won't approve before changing it!
• 🔥 burn it to the ground (aka old code, please remove it).
• ✅ LGTM: looks good to me!