Add topic join deps and fix Omni asset grouping for full lineage

[vbalalian]

Summary

• Parse Omni topic YAML files (omni/BigQuery/*.topic.yaml) to discover join dependencies that the Omni API doesn't expose, adding them to workbook asset specs so the full lineage graph connects correctly (e.g. Product Category Performance now shows deps on dim_products, dim_users, dim_user_rfm in addition to fct_events)
• Replace group_name: omni with tags: { layer: bi } in the Omni component translation config so workbook assets flow downstream from dbt models instead of appearing as isolated nodes in a separate group
• Update tests to cover topic YAML parsing and recursive join extraction

Test plan

• All 8 tests pass in dagster-project/tests/test_definitions.py
• dg list defs shows correct deps for all three workbooks after dg utils refresh-defs-state
• Verify in Dagster UI that Omni workbook assets flow downstream from dbt model assets
• Verify dim_products appears in Product Category Performance workbook lineage