Capture and translate primary key/identifier information between formats about incubator-xtable

[the-other-tim-brown]

@vamshigv can we close this as complete for now then?

[the-other-tim-brown]

Iceberg: https://iceberg.apache.org/javadoc/latest/org/apache/iceberg/Schema.html#identifierFieldIds--
Hudi: RecordKey
Delta: ?

[vamshigv]

For Delta generalized constraints like notnull, check, unique exists where unique may be close to PK and also only with Unity catalog there is support for PK support Ref https://docs.databricks.com/en/tables/constraints.html