[SPARK-29642][SS] Change the element type of underlying array to UnsafeRow for ContinuousRecordEndpoint

[HeartSaVioR]

What changes were proposed in this pull request?

This patch fixes the bug that ContinuousMemoryStream[String] throws error regarding ClassCastException - cast String to UTFString. This is because ContinuousMemoryStream and ContinuousRecordEndpoint uses origin input as it is for underlying data structure of Row, and encoding is missing here.

To force encoding, this patch changes the element type of underlying array to UnsafeRow instead of Any for ContinuousRecordEndpoint - ContinuousMemoryStream and TextSocketContinuousStream are modified to reflect the change.

Why are the changes needed?

Above section describes the bug.

Does this PR introduce any user-facing change?

No.

How was this patch tested?

Add new UT to check for availability on couple of types.

[HeartSaVioR]

cc. @tdas @zsxwing @jose-torres @gaborgsomogyi

[SparkQA]

Test build #112868 has finished for PR 26300 at commit f457f40.

• This patch fails Spark unit tests.
• This patch merges cleanly.
• This patch adds the following public classes (experimental):
• class ContinuousRecordEndpoint(buckets: Seq[Seq[UnsafeRow]], lock: Object)

[SparkQA]

Test build #112876 has finished for PR 26300 at commit 8cf2eba.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[vanzin]

Looks good as far as I can tell.

[vanzin]

retest this please

[SparkQA]

Test build #113289 has finished for PR 26300 at commit 8cf2eba.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[vanzin]

Merging to master.

[HeartSaVioR]

Thanks for reviewing and merging!