Implement pairing code

[unknownconstant]

Implement callback to expose pairing value to the Arduino sketch, and allow developer to implement confirm / reject behaviour.

[eltos]

This will also require

• a method to set the IO Capabilities
• Implementation of the different methods to be used in Phase 2
  • Just Works
  • Numeric Comparison
  • Passkey Entry
  • Out Of Band (OOB)

[unknownconstant]

@eltos The IO Capabilities could be set based off whether the callbacks have been implemented? i.e. if there is a callback for the pairing value then set the iocap to display, and if there is a confirm / reject implemented then set the input to yes/no etc.

Numeric comparison should be straightforward to implement, but I've not got capacity to do Passkey or Oob at present