[BUG] Double sorting in saved search cause csv report failure

[zhongnansu]

Describe the bug
When saved search is saved with double sorting, the logic fails to genereate csv. Single sorting doesn't have any issue though.

• Error message

{"statusCode":500,"error":"Internal Server Error","message":"_order.toLowerCase is not a function"}

• Root cause
  The following code will throw error if double sorting exists in the saved search
  • input(double sorting in metadata)

    "sort": [
        [
          "event.dateTime",
          "desc"
        ],
        [
          "client.user_id",
          "asc"
        ]
      ],
    

  • code with bug https://github.com/opensearch-project/dashboards-reports/blob/dev/dashboards-reports/server/routes/utils/dataReportHelpers.ts#L151-L160

To Reproduce
Steps to reproduce the behavior:

1. open a saved search in Discover
2. Add double sorting on 2 fields
3. save that search again

Expected behavior
report being generaeted with corrrect sorting

Plugins
Please list all plugins currently enabled.

Screenshots
If applicable, add screenshots to help explain your problem.

Host/Environment (please complete the following information):

• OS: [e.g. iOS]
• Version [e.g. 22]

Additional context
Add any other context about the problem here.