feat: tell of a newer release once a day, at a terminal

[achoimet]

With npm gone, nothing tells people a new CLI release is out: a downloaded binary stays as it is.

Behaviour

• At most once a day, the CLI asks where https://github.com/steadybit/cli/releases/latest redirects. That costs nothing against the GitHub API's rate limit. It caches the answer in ~/.steadybit/update-check.json.
• After the command, when a newer release exists, it prints to stderr:

  A new release of the Steadybit CLI is available: 6.0.0 → 6.0.1
  Update with: brew upgrade steadybit        (Homebrew install: the binary lives in a Caskroom)
  Get it from https://github.com/steadybit/cli/releases/latest    (otherwise)
  

• Silent when:
  • stderr isn't a terminal;
  • the CI, JENKINS_URL, TF_BUILD or BUILDKITE variables are set;
  • the command is completion or __complete;
  • the build isn't a release (go install …@main, dev builds);
  • STEADYBIT_NO_UPDATE_CHECK is set.
• The check runs alongside the command, which waits for it at most 1 s, once a day. Offline, the check is still recorded, so GitHub isn't asked again on every command.
• Versions compare by number, so 6.10.0 is newer than 6.9.0.

Testing

• Unit tests against a fake releases redirect, covering:
  • a newer release is announced;
  • at most one request per day, with the cached answer used in between;
  • silent when up to date, for non-releases, and when nobody would see it;
  • offline is recorded;
  • the number comparison.
• go test -race ./... and go vet (also GOOS=windows) pass.
• Live against GitHub with a build stamped 6.0.0, in a temporary home:
  • on a terminal it announces 6.0.1, and the second run uses the cache;
  • nothing is printed with stderr piped, with CI=true, or for completion;
  • the same binary run from a Caskroom path prints brew upgrade steadybit.