Send chat user id to ChatOps jobs

What does this MR do and why?

Send the chat user id of the user that triggered the Slack's slash command.

This will be used in this MR by ChatOps to mention the user on Slack when an end-to-end test pipeline is triggered. In future, this will also be used to mention the user on the end-to-end test session report incase there are failures.

Related to gitlab-com/chatops#110 (closed)

Closes #341798 (closed)

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