samples(discoveryengine): Updates to VAIS Data Store Samples

[holtskinner]

Description

• Change default behavior for GCS Import to be Unstructured PDFs (Users reported confusion)
• Add get/list data store samples
• Add purge data store sample

[snippet-bot]

Here is the summary of changes.

You are about to add 3 region tags.

• discoveryengine/get_data_store_sample.py:16, tag genappbuilder_get_data_store
• discoveryengine/list_data_stores_sample.py:16, tag genappbuilder_list_data_stores
• discoveryengine/purge_documents_sample.py:15, tag genappbuilder_purge_documents

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