Support shared catalog config across tables in iceberg-source

[lawofcycles]

Description

Adds support for a top level catalog configuration in iceberg-source that applies to all tables by default. When a table specifies its own catalog, it fully replaces the top level definition.

This reduces configuration duplication when multiple tables share the same catalog.

iceberg:
 catalog:
   type: rest
   uri: "http://iceberg-rest-catalog:8181"
   io-impl: "org.apache.iceberg.aws.s3.S3FileIO"
 tables:
    - table_name: "db.table_a"
     identifier_columns: ["id"]
    - table_name: "db.table_b"
     identifier_columns: ["id"]
     catalog:
       type: glue
       warehouse: "s3://other-bucket/warehouse"
       io-impl: "org.apache.iceberg.aws.s3.S3FileIO"

Issues Resolved

Partially addresses #6726 (catalog config sharing). Per-table shuffle overrides will be addressed after #6682 is merged.

Check List

• New functionality includes testing.
• New functionality has a documentation issue. Please link to it in this PR.
  • New functionality has javadoc added
• Commits are signed with a real name per the DCO

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