[SPARK-16199][SQL] Add a method to list the referenced columns in data source Filter

[rxin]

What changes were proposed in this pull request?

It would be useful to support listing the columns that are referenced by a filter. This can help simplify data source planning, because with this we would be able to implement unhandledFilters method in HadoopFsRelation.

How was this patch tested?

N/A

[SparkQA]

Test build #61209 has finished for PR 13901 at commit 846c0e4.

• This patch fails MiMa tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
  • abstract class Filter
  • case class EqualTo(attribute: String, value: Any) extends Filter
  • case class EqualNullSafe(attribute: String, value: Any) extends Filter
  • case class GreaterThan(attribute: String, value: Any) extends Filter
  • case class GreaterThanOrEqual(attribute: String, value: Any) extends Filter
  • case class LessThan(attribute: String, value: Any) extends Filter
  • case class LessThanOrEqual(attribute: String, value: Any) extends Filter
  • case class IsNull(attribute: String) extends Filter
  • case class IsNotNull(attribute: String) extends Filter
  • case class And(left: Filter, right: Filter) extends Filter
  • case class Or(left: Filter, right: Filter) extends Filter
  • case class Not(child: Filter) extends Filter
  • case class StringStartsWith(attribute: String, value: String) extends Filter
  • case class StringEndsWith(attribute: String, value: String) extends Filter
  • case class StringContains(attribute: String, value: String) extends Filter

[gatorsmile]

LGTM

[marmbrus]

No tests?