Preserve Iceberg column order on schema change

[lawofcycles]

resolves #531

Description

When columns are added to an Iceberg table via on_schema_change='sync_all_columns' or 'append_new_columns', they are currently always appended to the end of the table regardless of their position in the source model. This PR preserves the source column order by using ALTER TABLE ADD COLUMN ... AFTER instead of ALTER TABLE ADD COLUMNS.

For Iceberg/S3Tables incremental models, a new macro glue__process_schema_changes_with_column_order replaces the call to process_schema_changes in the incremental materialization. It walks the source column list to determine the correct position for each new column and issues individual ALTER TABLE ADD COLUMN col type AFTER prev_col (or FIRST) statements. Non-Iceberg models fall back to the existing process_schema_changes behavior.

Changes:

• dbt/include/glue/macros/adapters.sql: Add glue__process_schema_changes_with_column_order macro
• dbt/include/glue/macros/materializations/incremental/incremental.sql: Call the new macro for Iceberg schema changes
• tests/functional/adapter/test_iceberg.py: Add TestIcebergColumnOrderOnSchemaChange with single and multi-column addition test cases

Verified on a live AWS Glue environment that columns are inserted at the correct positions.

Checklist

• I have signed the CLA
• I have run this code in development and it appears to resolve the stated issue
• This PR includes tests, or tests are not required/relevant for this PR
• I have updated the CHANGELOG.md and added information about my change to the "dbt-glue next" section.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.

[akmalsoliev]

hey @lawofcycles,
does it happen to also cover the issue #673?
Thanks!