Add support for Prepare Write and Execute Write

[acouvreur]

When sending chunks of data that exceed the MTU we must chunk the data ourselves and rebuild it on the server side.

From my understanding, we can use the Prepare Write and Execute Write commands in order to send chunks of data at the protocol level.

It looks like some binding already have that capability: https://github.com/search?q=repo%3Atinygo-org%2Fbluetooth%20PREPARE&type=code

References:

• https://www.bluetooth.com/wp-content/uploads/Files/Specification/HTML/Core-61/out/en/host/attribute-protocol--att-.html#UUID-6715383c-a9e9-4cba-ebd4-84a866fb090d
• https://www.bluetooth.com/wp-content/uploads/Files/Specification/HTML/Core-61/out/en/host/attribute-protocol--att-.html#UUID-6715383c-a9e9-4cba-ebd4-84a866fb090d

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Maybe add the following function to DeviceCharacteristic

func (c DeviceCharacteristic) WritePrepare(offset uint16, p []byte) {
  ... 
}