Add PNP ID designation to DeviceInfoService

[ktnyt]

This PR introduces the PnP ID characteristic from the Device Information Service specification to DeviceInfoService.
Currently, there is no way to specify the Vendor ID and Product ID of the Bluetooth device using CircuitPython which can cause inconveniences when trying to make certain operating systems recognize the Bluetooth peripheral device as a specific product (e.g. an Xbox One S Wireless Controller in macOS).
The changes introduced by this PR will allow a developer to set a 8-bit value and three 16-bit values as illustrated below:

device_info = DeviceInfoService(
    software_revision=adafruit_ble.__version__,
    pnp_id=(0x02, 0x045E, 0x02E0, 0x0100),
)

The four values being the Vendor ID source field (8-bits), Vendor ID field (16-bits), Product ID field (16-bits), and Product Version field (16-bits) respectively.

[tannewt]

Thank you! I'd love to see a write up on adafruit-playground.com about mimicking an xbox controller.

[dhalbert]

@ktnyt While working on #204 I also discovered that if pnp_id is not supplied, an exception is raised because the value is None, and that doesn't unpack into the StructCharacteristic. What should be the default value pnp_id when it's not needed. Should all the fields be zeros, or something else?

[ktnyt]

@dhalbert I'm sorry I couldn't catch that. I believe that zeros should be fine since the specification doesn't elaborate on what values are valid.