Support FIRST/AFTER keywords for ALTER TABLE MODIFY COLUMN in Iceberg tables #2091

The main conversation started over at #1841 (comment) where the PR accidentally went from a bug fix to a full blown feature.

Basically, this issue exists to keep track of the currently-missing FIRST/AFTER support when we were adding general ALTER support for directly modifying Iceberg tables.

E.g.

ALTER TABLE ice.`ns1.table1`
    (MODIFY COLUMN `col2` Nullable(Int32) FIRST );

where table1 is an iceberg table.