Add GAP/GATT Callbacks

[microbuilder]

In terms of callbacks to make life easier for customers, we should have optional connected and disconnected callbacks as a minimum, which are easy to implement in peripheral mode since we only have one connection to worry about.

The more complicated question is how to handle GATT callbacks.

In the case of wrapper classes for things like BLE UART or HID Keyboard, this is easy since we can expose a callback specific to that class in the helper, but for user-defined GATT characteristics we'll need some sort of generic callback with a way to get the handle of the characteristic that caused the CB, etc.

• Add connectedcallback (peripheral mode)
• Add disconnected callback (peripheral mode), ideally including the reason (user disconnected, connection timed out, etc.)
• Add bleuart_rx callback (in the bleuart helper class)
• Define a generic gatt callback for user-defined characteristic changes, allowing easy access to the relevant characteristic data