materialize-s3-iceberg: sort tables by collection key on create

[jacobmarble]

Description:

When materialize-s3-iceberg creates a new Iceberg table, set its write sort order to the collection-key fields (identity transform, ascending, nulls-first). Previously tables were created unsorted.

Workflow steps:

New tables created after this lands will carry a sort order in their Iceberg metadata; existing tables are untouched.

Documentation links affected:

None.

Notes for reviewers:

• Identifier fields are deliberately not set: this connector is delta-updates only and append-only, so it cannot guarantee row-level uniqueness, which Iceberg's identifier_field_ids implies.
• Field IDs are assigned 1..N by iceberg-ctl over FieldSelection.AllFields() (Keys ++ Values ++ Document), so the key field IDs are simply 1..len(Keys).
• Verified end-to-end via the existing TestIntegration against the dockerized Polaris REST catalog.