Add deploy chat command

What does this MR do?

This adds a new ChatOps command:

/trigger deploy <environment> to <environment>

Does this MR meet the acceptance criteria?

• Changelog entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

cc @zj

Added 1 commit:

• 844ba3b9 - Add class for describing the execution result [ci skip]

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Added 1 commit:

• 1d16f137 - Add deploy chat command [ci skip]

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Mentioned in merge request gitlab-com/www-gitlab-com!3851 (merged)

Marked the task Changelog entry added as completed

Marked the task API support added as completed

Marked the task Added for this feature/bug as completed

Added 1 commit:

• a36d5561 - Introduce deploy command that allows to start deployment from one environment to second one

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Resolved all discussions

Added ~122770 label

Reassigned to @ayufan