Add trigger_source to agent presence

What does this MR do and why?

Add trigger_source to agent presence, which is needed to decide if a flow was started by a human, or some system/scheduled trigger when displaying the agent presence on a comment.

References

Followup to !257097 (merged)

#629909 (closed)

How to set up and validate locally

1. Run a developer flow.
2. Run this graphQL query. triggerSource should show as HUMAN:

query notePresence {
  note(id: "gid://gitlab/Note/11698") {
    id
    body
    author { 
      id
      username
    }
    agentPresence {
      sessionId
      agentName
      agentCatalogWebPath
      sourceType
      sourceLink
      triggerSource
      user {
        id
        username
        name
        avatarUrl
        webPath
      }
      userPermissions {
        readDuoWorkflow
      }
    }
  }
}

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