This PR adds a `private[sql]` method `metadata` to `SparkPlan`, which can be used to describe detail information about a physical plan during visualization. Specifically, this PR uses this method to provide details of `PhysicalRDD`s translated from a data source relation. For example, a `ParquetRelation` converted from Hive metastore table `default.psrc` is now shown as the following screenshot:

![image](https://cloud.githubusercontent.com/assets/230655/11526657/e10cb7e6-9916-11e5-9afa-f108932ec890.png)

And here is the screenshot for a regular `ParquetRelation` (not converted from Hive metastore table) loaded from a really long path:

![output](https://cloud.githubusercontent.com/assets/230655/11680582/37c66460-9e94-11e5-8f50-842db5309d5a.png)

Author: Cheng Lian <lian@databricks.com>

Closes #10004 from liancheng/spark-12012.physical-rdd-metadata.