SchemaBuilder improvements

SchemaBuilder code will be moved to the exec package from test in DRILL-6901.
There are a couple of improvements that can be done in the existing code:

1. ColumnBuilder: setScale method
setScale method is a bit awkward, it requires precision as second parameter. More natural to go with precision and then scale.
Suggestion is to have setPrecisionAndScale method instead which will accept precision and scale as first and second parameters.
https://github.com/apache/drill/blob/master/exec/java-exec/src/test/java/org/apache/drill/test/rowSet/schema/ColumnBuilder.java#L57

2. SchemaContainer: addColumn method
addColumn method has parameter AbstractColumnMetadata, since we have interface ColumnMetadata, it's better to operate on the interface level rather than on the abstract class.
https://github.com/apache/drill/blob/master/exec/java-exec/src/test/java/org/apache/drill/test/rowSet/schema/SchemaContainer.java#L28

3. MapBuilder / RepeatedListBuilder / UnionBuilder: buildCol method
buildCol method is private in these classes. These classes create columns and add them to the schema. There might be use cases when dev needs only column and will add it to the schema when needed. Suggestion is to make buildCol method public. Also all these classes require SchemaContainer as parent, though when we only need them to build the column, second constructor can be added without SchemaContainer parameter.