[SPARK-18721][SS] Fix ForeachSink with watermark + append

## What changes were proposed in this pull request?

Right now ForeachSink creates a new physical plan, so StreamExecution cannot retrieval metrics and watermark.

This PR changes ForeachSink to manually convert InternalRows to objects without creating a new plan.

## How was this patch tested?

`test("foreach with watermark: append")`.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #16160 from zsxwing/SPARK-18721.