Added basic hid gamepad example

[fluffymadness]

Got my changes to work for a basic Bluetooth hid gamepad. Tested under Linux/Windows10/Android.
The dummy input report sends two pressed buttons, and the analog stick is pushed in the down-right corner.

[mringwal]

Thanks for the demo. Any idea on how to make it more 'fancy'?

[fluffymadness]

Hmm, not really.
I was just thinking about it as a starting point for people trying to make a custom gamepad.
So the hardcoded values would be replaced with the input data from the buttons.
But I'd like to add xinput, switch pro controller emulation.

I did some tests with two hid descriptors that I got to work with
nkolban's bluetooth library for the esp32. They got detected and loaded in windows. (BLE HID)
https://github.com/nkolban/esp32-snippets/tree/master/cpp_utils

But right now when trying to use these longer hid descriptors with btstack I get stacktraces
when loading them.
I would open another issue on that with proper logs.

[fluffymadness]

longer descriptors are working without problem, it was a issue in my code.