Read RSSI of connected device

[TNorbury]

resolves #292 Adds the ability to read the RSSI of a connected device. While the issue did ask for an RSSI stream I decided to go with just a Future instead. The reason being is the native BLE layers (to the best of my knowledge) don't have a mechanism for streaming RSSI, just reading the value once per call, as such I thought that it would be best to maintain that in the Flutter-side API.

I also updated the example app to include a Read RSSI button, mostly just as a proof of functionality, so it can be removed if need-be, you can see that demonstrated below:

Android

iOS