Make 'collapse_root' default Spark schema generation option

[yruslan]

Background

spark-cobol supports

.option("schema_retention_policy", "collapse_root")

and

.option("schema_retention_policy", "keep_original")

Currently, 'keep_original' is the default one. But 'collapse_root' is more often used since it is easier to apply transformations in Spark when fields are not nested.

Feature

Make

.option("schema_retention_policy", "collapse_root")

the default option and use it if none are specified