[iceberg] Avoid partition evolution when partition fieldId is not 0

[muttcg]

Purpose

Linked issue: close #7068

Handles fieldId incompatibility between Paimon (starts at 0) and Iceberg (starts at 1).

Iceberg REST schema creation shifts all fieldIds by +1, causing field disorder. While schemas can be updated post-creation to start at fieldId 0, creating an empty schema first triggers partition evolution issues that break some query engines, making them believe the partition has evolved and that queries must account for this.

Strategy based on partition field position in the schema:

• Position 0: Creates empty schema first, because partition evolution unavoidable
• Position > 0: Creates dummy schema with offset fields and gap filling to preserve the partition spec

Tests

Added new unit test IcebergRestMetadataCommitterTest.testPartitionedPrimaryKeyTableWithNonZeroFieldId
Made tests with Polaris REST catalog and StarRocks

Docs

Added section to rest-catalog.md explaining changes