[SPARK-15593][SQL] Add DataFrameWriter.foreach to allow the user consuming data in ContinuousQuery

## What changes were proposed in this pull request?

* Add DataFrameWriter.foreach to allow the user consuming data in ContinuousQuery
  * ForeachWriter is the interface for the user to consume partitions of data
* Add a type parameter T to DataFrameWriter

Usage
```Scala
val ds = spark.read....stream().as[String]
ds.....write
         .queryName(...)
        .option("checkpointLocation", ...)
        .foreach(new ForeachWriter[Int] {
          def open(partitionId: Long, version: Long): Boolean = {
             // prepare some resources for a partition
             // check `version` if possible and return `false` if this is a duplicated data to skip the data processing.
          }

          override def process(value: Int): Unit = {
              // process data
          }

          def close(errorOrNull: Throwable): Unit = {
             // release resources for a partition
             // check `errorOrNull` and handle the error if necessary.
          }
        })
```

## How was this patch tested?

New unit tests.

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #13342 from zsxwing/foreach.