Add an API endpoint to check if an update is available

[Senyoret1]

Feature description
There should be an API endpoint for knowing if there is an update available. This is because the manager UI currently can show a button for updating the visor, but not a visual clue for letting the user know when to use it.

Is your feature request related to a problem? Please describe.
No, it would be for an UI/UX improvement.

Describe the solution you'd like
A simple API endpoint returning if an update is available would be enough. However, if the endpoint can return the number of the current version and the new version it would be even better.

Describe alternatives you've considered
Maybe the front-end could check the lastest version using a method similar to the one used by the Go code, but that would mean repeated code, so if a change is made in the Go code and the same change is not made inmediatelly on the front-end code, problems may appear.

Additional context

Possible implementation