docs: add BigLake Iceberg metastore integration guide

[GuyEshdat]

Summary

• Adds documentation for the new BigLake Iceberg metastore integration
• Covers required IAM roles including roles/serviceusage.serviceUsageConsumer (needed for the BigLake REST API quota attribution, not included in roles/biglake.metadataViewer)
• Documents the GCS storage condition for scoping Storage Object Viewer
• Includes setup steps for Glue and BigLake connections

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