Add Write command to the darwin gattc implementaiton

[Fabianexe]

This solve the Darwin part of #153.

I have tested it and it works like expected.

[aykevl]

Checked the documentation and this just sends the write, it apparently doesn't wait for a response so the caller doesn't know whether the call succeeded or not, or even whether the BLE connection is still alive. It always returns nil. Perhaps it should wait until the didWriteValueFor event is received?
For more information, see:
https://pkg.go.dev/github.com/tinygo-org/cbgo#Peripheral.WriteCharacteristic
https://developer.apple.com/documentation/corebluetooth/cbperipheral/1518747-writevalue

[Fabianexe]

@aykevl you are right I have not looked so deep into the documentation, sorry for that.

I have added the delegate and wait now if an error appears.

Writing, waiting, and timeouts work fine on my mac.