[SPARK-11216][SQL][follow-up] add encoder/decoder for external row

[cloud-fan]

address comments in #9184

[SparkQA]

Test build #44120 has finished for PR 9212 at commit 705dbfb.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * case class CreateExternalRow(children: Seq[Expression]) extends Expression\n

[cloud-fan]

cc @marmbrus

[marmbrus]

Thanks, merging to master.