feat(android): implement split tunneling (per-app proxy) support

[SteeaaN]

Related Issue

Relates to TrustTunnel/TrustTunnelFlutterClient#27
Partially addresses #24 (This implements application-based split tunneling for Android).

Summary

This PR implements application-based split tunneling (per-app proxy) specifically for the Android platform. It allows defining an inclusion or exclusion list of Android package names to route through the VPN.

Note: A corresponding PR implementing the UI for this feature will be opened in TrustTunnelFlutterClient.

Changes

• Added perAppProxy (bool), bypassApps (bool), and proxyApps (list) properties to the Kotlin VpnServiceConfig to receive configuration from the Flutter plugin.
• Modified VpnService.kt to apply addAllowedApplication and addDisallowedApplication rules to the Android VpnService.Builder during tunnel creation.

Tests

• New or updated tests cover the changes
• All existing tests pass (make test)
• Benchmarks updated (if performance-relevant)

Checklist

• Documentation updated (if applicable)
• CHANGELOG.md updated (if user-visible change)
• No secrets or credentials committed