DataSourceV2 should provide a way to get a source's schema.

To validate writes with DataSourceV2, the planner needs to get a source's schema. The current API has no direct way to get that schema. SPARK-23321 instantiates a reader to get the schema, but sources are not required to implement ReadSupport or ReadSupportWithSchema. V2 should either add a method to get the schema of a source, or require sources implement ReadSupport.