support update、delete、merge sql

[melin]

The big advantage of iceberg is that it supports delete and update, and all operations are performed through sql to make it easy for users to use.

[hbutani]

there is no support for delete, update for datsources v1 in spark 2.x
This is coming in Spark 3.0 for datasources v2.
For Spark 2.x you can simulate an update/delete by doing an insert overwrite.

[melin]

Refer to this method to extend the sql syntax：
delta-io/delta@b9cf201