[BUG] Data source reference is removed when adding sample data

[BionIT]

Describe the bug

In add sample data page when data source feature is turned on, the data source is added as reference to the index pattern. However, the behavior is lost due to https://github.com/opensearch-project/OpenSearch-Dashboards/pull/6105/files#diff-fcdd5446e0a0ea3950cf54e2c6b9fee8011c5429016a8992380c62339e4d051fL63

To Reproduce
Steps to reproduce the behavior:

1. Turn on data source
2. Go to add sample data
3. select a data source from picker.
4. click on import
5. go to dashboard management, then index pattern, and the data source connection is lost

Expected behavior
Data source should be the reference of the index pattern

OpenSearch Version
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Dashboards Version
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Plugins

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Screenshots

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Host/Environment (please complete the following information):

• OS: [e.g. iOS]
• Browser and version [e.g. 22]

Additional context

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[wanglam]

Hi @BionIT , Thank you for reporting this bug. I think the root cause is removed the data source reference by accident in this PR. Will add it back and add some unit tests in these code.