samples: net: openthread: Add a sample using OpenThread CoAP API

In order to show how to use OpenThread and CoAP, add an application.
This example could be build to run a client or server.
The server could expose LEDs and buttons and client could get their
state and set the LEDs state.

The network is created automatically using the network key predefined
in the config. The goal is to make the example simple by removing
the commisionning and joinning process.

If a client application has a button, it could use it to toggle the first
LED on any boards running the server application.

Signed-off-by: Alexandre Bailon <abailon@baylibre.com>