Added mtuSize variable for BluetoothPeripheral

[carillem]

After the changeMTU call, mtuSize is returned as a string but not stored in BluetoothPeripheral. This change allows the user to access the negotiated mtuSize in the BluetoothPeripheral class.

EDIT: After a lot of internet sleuthing, the function name for the CoreBluetooth CBPeripheral library turned out to be maximumWriteValueLengthForType: https://forums.developer.apple.com/forums/thread/73871. Also updated for int conversion.

[Reedyuk]

looks good but you will need to put in the actuals for other platforms too

jsMain and api main. - Given the nature of this change, just put the actuals in, you dont need to implement the method/updates.

[Reedyuk]

Seems to be failing on the CI.

e: file:///Users/runner/work/blue-falcon/blue-falcon/library/src/appleMain/kotlin/dev/bluefalcon/BlueFalcon.kt:183:43 Unresolved reference 'maximumWriteValueLength'.
e: file:///Users/runner/work/blue-falcon/blue-falcon/library/src/appleMain/kotlin/dev/bluefalcon/BlueFalcon.kt:183:67 Only expressions are allowed here.
e: file:///Users/runner/work/blue-falcon/blue-falcon/library/src/appleMain/kotlin/dev/bluefalcon/BlueFalcon.kt:183:70 Syntax error: Expecting '(' to open a loop range.
e: file:///Users/runner/work/blue-falcon/blue-falcon/library/src/appleMain/kotlin/dev/bluefalcon/BlueFalcon.kt:183:72 Syntax error: Expecting an expression.
e: file:///Users/runner/work/blue-falcon/blue-falcon/library/src/appleMain/kotlin/dev/bluefalcon/BlueFalcon.kt:183:73 Unresolved reference 'withResponse'.

[carillem]

Seems to be failing on the CI.

e: file:///Users/runner/work/blue-falcon/blue-falcon/library/src/appleMain/kotlin/dev/bluefalcon/BlueFalcon.kt:183:43 Unresolved reference 'maximumWriteValueLength'.
e: file:///Users/runner/work/blue-falcon/blue-falcon/library/src/appleMain/kotlin/dev/bluefalcon/BlueFalcon.kt:183:67 Only expressions are allowed here.
e: file:///Users/runner/work/blue-falcon/blue-falcon/library/src/appleMain/kotlin/dev/bluefalcon/BlueFalcon.kt:183:70 Syntax error: Expecting '(' to open a loop range.
e: file:///Users/runner/work/blue-falcon/blue-falcon/library/src/appleMain/kotlin/dev/bluefalcon/BlueFalcon.kt:183:72 Syntax error: Expecting an expression.
e: file:///Users/runner/work/blue-falcon/blue-falcon/library/src/appleMain/kotlin/dev/bluefalcon/BlueFalcon.kt:183:73 Unresolved reference 'withResponse'.

Just updated the PR with the change. The maximumWriteValueLength belongs to CBPeripheral based on their documentation: https://developer.apple.com/documentation/corebluetooth/cbperipheral/maximumwritevaluelength(for:). Hopefully this resolves it.