Add list-trigger and trigger subcommands

[4thel00z]

Description

A subcommand for listing triggers and using triggers via glab is missing.
Added these two subcommands for that.
Background is that I have manual action I want to trigger, and using a combination of rules that check for CI variable and this subcommand it is possible to trigger manual actions.

Related Issue

This is kinda related to this issue: #939

How Has This Been Tested?

Running list-trigger against an internal repo.

Screenshots (if appropriate)

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation
• Chore (Related to CI or Packaging to platforms)

[clemsbot]

This PR has been automatically marked as stale because it has not had recent activity. This message is just a reminder for the author, reviewers and assignees.

[profclems]

Hi @4thel00z, thanks a lot for this PR. We announced that this project is officially adopted by GitLab #983 (comment). All new contributions should be mode to the GitLab repo: https://gitlab.com/gitlab-org/cli

[clemsbot]

This PR has been automatically marked as stale because it has not had recent activity. This message is just a reminder for the author, reviewers and assignees.