Refactor updater API

[cmaglie]

Please check if the PR fulfills these requirements

• The PR has no duplicates (please search among the Pull Requests
  before creating one)
• Tests for the changes have been added (for bug fixes / features)

• What kind of change does this PR introduce?
  This PR refactor the updater package, it aims to provide a more defined API for the autoupdate feature.
  The API defined here is composed of two function that the auto-updater should implement:

// Start checks if an update has been downloaded and if so returns the path to the
// binary to be executed to perform the update. If no update has been downloaded
// it returns an empty string.
func Start(src string) string {
	return start(src)
}

// CheckForUpdates checks if there is a new version of the binary available and
// if so downloads it.
func CheckForUpdates(currentVersion string, updateAPIURL, updateBinURL string, cmdName string) (string, error) {
	return checkForUpdates(currentVersion, updateAPIURL, updateBinURL, cmdName)
}

we can differentiate the auto-update procedure for different OS simply by implementing these two functions for each OS via go:build flags, as already demonstrated here for macos here (for now it's just an empty stub).

• What is the current behavior?
  No changes

• What is the new behavior?

• Does this PR introduce a breaking change?

• Other information: