Update to Flutter 2.0 with dart 2.12.0 (nullsafety)

• Update to Flutter 2.0 with dart 2.12.0
• Example Android in Flutter Embedded V2

Deployment target is iOS9.0

• According the CHANGELOG of CryptoSwift, the minimum deployment target is iOS9.0
  (https://github.com/dokinkon)

Http method HEAD/GET

Add http method to verify certificate

Fix Android

• Fix Kotlin version
• Fix MissingPluginException(No implementation found for method close on channel ssl_pinning_plugin)
• Code cleanup

Update version

• Update to v2 android plugin APIs
• Kotlin version is out of date
• swift code version not provided

ssl_pinning_plugin

Plugin for check SSL Pinning on request HTTP.

Checks the equality between the known SHA-1 or SHA-256 fingerprint and the SHA-1 or SHA-256 of the target server.