refactor(wifi_scan): Federated plugin architecture

[Alystrasz]

This pull request restructures the repository to follow the federated plugin format:

• WiFiScan now is the app-facing class, and as such contains package methods signatures (canStartScan, startScan etc);
• Existing code was moved in MethodChannelWifiScan class, which now is a WiFiScan implementation.

This change will make it easier to add support for new platforms (e.g. Linux).