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

[cloud-fan]

Implement encode/decode for external row based on ClassEncoder.

TODO:

• code cleanup
• fix corner cases
• refactor the encoder interface
• improve test for product codegen, to cover more corner cases.

[SparkQA]

Test build #43991 has finished for PR 9184 at commit 4751418.

• This patch fails to build.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * case class CreateRow(children: Seq[Expression]) extends Expression\n

[marmbrus]

Lets put this in its own test suite.

[marmbrus]

This is looking great! Thanks for coming up with a draft so quickly.

[SparkQA]

Test build #44058 has finished for PR 9184 at commit b08da16.

• This patch fails Scala style tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):\n * case class ClassEncoder[T](\n * case class CreateRow(children: Seq[Expression]) extends Expression\n

[SparkQA]

Test build #44061 has finished for PR 9184 at commit 4551a92.

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

[marmbrus]

Is this really the right thing to do? I think we are going to NPE execution. If anything I think we'd want to null out all the columns, though I'm not sure we need to handle this case at all.

[marmbrus]

This looks great. I'm going to merge so that I can resolve any conflicts with #9190. We can continue to address comments / TODOs in a follow up PR.