Support PIN authentication

[Cheprod]

Hi,

I work with BLED112 Dongle to connect on BLE Device.

I need to make an authentication with PIN Code (a fixed one). I found some information about this subject especially here : https://github.com/peplin/pygatt/issues/124

Maybe things evolve since 2017 ;)

If Pygatt doesn't want to support this feature, I'm ready to implement it in a fork.

[peplin]

I'd be happy to merge a pull request with this feature.

[manjunathmj]

@peplin any progress on PIN based authentication?