Change ScanResult#isConnectable from nullable boolean to non-nullable enum

[mikolak]

https://github.com/Polidea/FlutterBleLib/pull/586#discussion_r610065825

Connectivity with 3 states: Connectivity.noInfo (or notAvailable?), Connectivity.notConnectable, Connectivity.Connectable

[okocsis]

Right now it's Bool? isConnectable so ternarily [null, false, true] it's nullable because it's only available on iOS host OS, on android it is still connectable so null very importantly does not mean false at all! Therefore I'd also recommend using a less misleading representation being the enum Connectivity [ .notAvailable| .unsupported, . notConnectable, .connectable] correspondingly mapping to the Bool? values above.