[SPARK-22909][SS]Move Structured Streaming v2 APIs to streaming folder

[zsxwing]

What changes were proposed in this pull request?

This PR moves Structured Streaming v2 APIs to streaming folder as following:

sql/core/src/main/java/org/apache/spark/sql/sources/v2/streaming
├── ContinuousReadSupport.java
├── ContinuousWriteSupport.java
├── MicroBatchReadSupport.java
├── MicroBatchWriteSupport.java
├── reader
│   ├── ContinuousDataReader.java
│   ├── ContinuousReader.java
│   ├── MicroBatchReader.java
│   ├── Offset.java
│   └── PartitionOffset.java
└── writer
    └── ContinuousWriter.java

How was this patch tested?

Jenkins

[zsxwing]

cc @jose-torres @marmbrus @cloud-fan

[jose-torres]

LGTM

[SparkQA]

Test build #85437 has finished for PR 20093 at commit 0e67a0b.

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

[cloud-fan]

LGTM, merging to master!