Add teleport status --is-up-to-date

[hugoShaka]

This PR adds a flag --is-up-to-date to the teleport-update status command. In some specific setups, users don't have systemd but still want to write their own logic to update and restart Teleport. See this Slack thread for more context.

While we cannot cover every use-case, we can make it easy for users to detect if an update is required. They will then be responsible for triggering the update and reloading/restarting the teleport processes.

Changelog: Adds the the teleport-update status --is-up-to-date flag to change the return code based on the update status.

[vapopov]

I would add a test for this command func Run(args []string) int

[hugoShaka]

I would add a test for this command func Run(args []string) int

I'm not a huge fan of testing the whole status thing as it's going to require a full blown mock environment, with a fake config, proxy mock, ... This is basically an integration test and should be in integration/

One quick way to add coverage would be to extract a small function:

func statusExitCode(ccfg cliConfig, status agent.Status) int {
    // ...
}

Would this work for you?

[backport-bot-workflows]

@hugoShaka See the table below for backport results.

Branch	Result
branch/v15	Create PR
branch/v16	Create PR
branch/v17	Create PR
branch/v18	Create PR