Add subcommand for canceling export jobs

[gabor-boros]

Summary

At the moment, the mattermost client already knows how to cancel a job, but mmctl has no subcommands to expose this functionality and cancel an export job.

This PR adds a new subcommand mmctl export job cancel [export job id] to cancel an export job.

Ticket Link

This PR is not resolving any open issues in Jira or GitHub. If necessary, I can create a GitHub issue to be able to link the PR to that.

Testing instructions

1. Create a new export job
2. Get the export job's ID
3. Execute mmctl export job cancel <ID>

First, the export job's state should be cancel_requested, then it should switch to canceled after some time.

Example

# Create and cancel the job
$ ./mmctl export create --attachments \
      | awk -F": " '{print $2}' \
      | xargs -n1 -I{} ./mmctl export job cancel "{}"

# Get job list again
$ ./mmctl export job list
  ID: 59uow53d53f6byk5a4hbwg6y4c
  Status: canceled
  Created: 2022-09-20 08:21:55 +0200 CEST

  ID: kw1kppccy7b7dnoqzdayh9q4jw
  Status: success
  Created: 2022-09-20 08:21:39 +0200 CEST
  Started: 2022-09-20 08:21:40 +0200 CEST
  Data: map[include_attachments:true]

Documentation

Although it was not completely necessary to update the mmctl export job show subcommand's example usage, I did it to have a more consistent documentation.

[mattermod]

Hello @gabor-boros,

Thanks for your pull request! A Core Committer will review your pull request soon. For code contributions, you can learn more about the review process here.

[mattermod]

This PR has been automatically labelled "stale" because it hasn't had recent activity. A core team member will check in on the status of the PR to help with questions. Thank you for your contribution!

[gabor-boros]

/update-branch

[gabor-boros]

@isacikgoz Thank you for your review! Your observations are addressed as part of 65daf40. Could you please do another round of review?

Also, could you please guide me on how I could help out at https://github.com/mattermost/mattermost-server/pull/21049 to get it merged? 😇

[gabor-boros]

/update-branch

[isacikgoz]

@gabor-boros can you update the docs with make docs command? The CI is failing due to that error.

[gabor-boros]

Ah, sure @isacikgoz. Give me a day or two. 😌

[gabor-boros]

FYI: @isacikgoz updated