add-on: troubleshoot: add file transfer support

[joelguittet]

The purpose of this ticket is to integrate Device Troubleshoot File Transfer API. File Transfer in bot direction, from the device to the server and from the server to th device should be supported. This can be done with a litllefs partition for demonstration purpose.

Tasks includes:

• integration of the File Transfer messages
• integration of the Control messages (dependency of File Transfer)
• creation of File Transfer API
• update of examples to show the usage of the Device TroubleShoot File Transfer API
• update of the documentation

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

• callback function when to save a file sent by the server
• callback function to read a file from the device

Note: there is no documentation of the protocol used by the Device Troubleshoot add-on. Retro-engineering is done to understand how the existing mender solution is working.

References:

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

[joelguittet]

Draft version is available on branch at the moment for evaluation purpose on esp-idf platform. It has been tested with files up to 100KB in both upload and download directions. Curl support that is used for convenient development is tested as well and working. Zephyr support is not tested at all and will be checked soon to ensure it is working too.

[joelguittet]

Zephyr support has been added with some websocket fixes. PR is no more draft and feature will be integrated soon.

[joelguittet]

PR has been merged.