add Timestamp.MICROS for iceberg timestamptz

[aIbrahiim]

Fixes PreCommit/PostCommit SQL failures caused by #39344, which maps Iceberg timestamptz to Beam Timestamp.MICROS

Beam SQL did not handle that logical type, and one Iceberg SQL test compared the wrong schemas (SQL CREATE still uses DATETIME; Iceberg reload now returns Timestamp.MICROS).

#35514
#34314

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[aIbrahiim]

cc: @ahmedabu98

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[ahmedabu98]

Thanks @aIbrahiim, I left a couple comments.

I think we should also add a TODO somewhere to make the Timestamp logical type the new default for SQL timestamps instead of primitive FieldType.DATETIME

[ahmedabu98]

maybe rename to testSqlTimestampLogicalType because the test value is in millis precision?