Prompt installing/updating rzk when not found in PATH

[aabounegm]

If there is no rzk executable found in the PATH, provide an option (through a prompt displayed automatically to the user) to download the latest (supported) version from GitHub releases.

This also involves storing the supported rzk version range in the extension. Additionally, a prompt should be displayed to update when a newer (supported) version than the one in use is found on GitHub.

The priority for which binary to use should be as follows:

• The path provided in the settings (depends on #19)
• The global one on the PATH
• The version downloaded locally for the extension