Introduce per-characteristic max_write_without_response_size

[dlech]

A while back, we added a BleakClient.mtu_size property. This can be used to determine what is the largest size of data you can send using write without response which is mtu_size - 3.

There are a couple problems with this. One is that you have to know what MTU means and that you have to subtract 3 from it. The other is that the BlueZ backend doesn't currently doesn't have a proper way to get the negotiated MTU and defaults to the minimum value of 23.

However, BlueZ recently introduced a new MTU property on characteristics. BlueZ's reason for doing this is that with Bluetooth 5 there are enhanced attributes that allow different MTU sizes per characteristic.

So, I would like to propose adding a new max_write_without_response_size property on BleakGATTCharacteristic objects that returns mtu_size - 3, or in the case of BlueZ, the "MTU" d-bus property minus 3. This makes it obvious what the value should be used for and will hopefully be future-proof if other OSes add support for enhanced attributes.

[dlech]

implemented in #962