[devices] Implement an update mechanism for devices

[frgfm]

In order to avoid requiring ssh access to our devices, it would be handy for them to check whether they need to update and automatically do it if necessary.

Context

• the device needs to be able to check which software/docker version it needs
• depending on the result of the comparison with its current version, it might need to go download the software (outside of the scope of this repo)

Implementation suggestion

I gave it some thoughts and here is what I suggest:

• similarly to the specs field (which we should rename to hardware_specs), we introduce a software_hash field.
• the corresponding value will be a deterministic result for a given software config (let's say the docker image hash, or the commit hash)
• just like devices that will ping regularly, the device will come check the software_hash. If it's different from the one it uses, then it updates.

I strongly suggest going for the commit hash, at least to check the mechanism

What do you think @pyronear/back-end ?