[SPARK-12719][SQL] SQL generation support for Generate

## What changes were proposed in this pull request?

This PR adds SQL generation support for `Generate` operator. It always converts `Generate` operator into `LATERAL VIEW` format as there are many limitations to put UDTF in project list.

This PR is based on https://github.com/apache/spark/pull/11658, please see the last commit to review the real changes.

Thanks dilipbiswal for his initial work! Takes over https://github.com/apache/spark/pull/11596

## How was this patch tested?

new tests in `LogicalPlanToSQLSuite`

Author: Wenchen Fan <wenchen@databricks.com>

Closes #11696 from cloud-fan/generate.