add-on: integration of Device Troubleshoot

[joelguittet]

The purpose of this ticket is to integrate Device Troubleshoot API.
Particularly the ability to have a remote terminal on the device from the mender server should be demonstrated. This can be done using mender-stm32l4a6-zephyr-example to access the Zephyr console.

Tasks includes:

• integration of the Device Connect API
• integration of websocket API by renaming platform "http" directory to "net"
• creation of the troubleshoot add-on (will be optional based on build configuration)
• update of examples to show the usage of the Device TroubleShoot API
• update of the documentation

The build of the Device Troubleshoot add-on the MCU must provides:

• option to include the add-on

The API of the Device Troubleshoot add-on the MCU must provides:

• function to connect/disconnect the Troubleshoot add-on to the mender server
• callback function when data are received from the end user and API to print data on the terminal
• callback function to be informed of the shell status (connection and disconnection of the end user)

References:

• https://mender.io/product/features/device-troubleshooting
• https://docs.mender.io/api/#device-api-device-connect