Part of #220.

This PR adds a new release check which is used in two different ways:

• A new stackablectl version check command can be used to check if the current installation is up-to-date
• A check automatically runs for all other commands (in parallel to not block the commands) and the status is reported as part of the final command output.

This PR has a few known issues, mostly centered around output rendering/composition. This should eventually be reworked.

There will be a follow-up PR, which introduces a config file for stackablectl to disable running the release check (and other settings in the future).

Regarding the goals stated in #220 (comment):

Goals

• Checks if a new version is available (yes/no)
• Report to STDERR as suggested
• Gracefully handles offline/no internet (just skip the check, no error, should not block): The execution of commands is unhindered by any failure of the check. If the check couldn't be performed, no output is printed.
• Use the GitHub releases API to get information

Maybe

• Cache the result or record the last checked timestamp and only check once a week or once a day: This uses the same caching mechanism already in place. In the future, we might want to implement different cache durations and make them configurable through the upcoming config file.
• Opt-out mechanism: Will be handled in a follow-up PR as mentioned above.