[SPARK-26536][BUILD][FOLLOWUP][TEST-MAVEN] Make StreamingReadSupport public for maven testing

## What changes were proposed in this pull request?

`StreamingReadSupport` is designed to be a `package` interface. Mockito seems to complain during `Maven` testing. This doesn't fail in `sbt` and IntelliJ. For mock-testing purpose, this PR makes it `public` interface and adds explicit comments like `public interface ReadSupport`

```scala
EpochCoordinatorSuite:
*** RUN ABORTED ***
  java.lang.IllegalAccessError: tried to
access class org.apache.spark.sql.sources.v2.reader.streaming.StreamingReadSupport
from class org.apache.spark.sql.sources.v2.reader.streaming.ContinuousReadSupport$MockitoMock$58628338
  at org.apache.spark.sql.sources.v2.reader.streaming.ContinuousReadSupport$MockitoMock$58628338.<clinit>(Unknown Source)
  at sun.reflect.GeneratedSerializationConstructorAccessor632.newInstance(Unknown Source)
  at java.lang.reflect.Constructor.newInstance(Constructor.java:423)
  at org.objenesis.instantiator.sun.SunReflectionFactoryInstantiator.newInstance(SunReflectionFactoryInstantiator.java:48)
  at org.objenesis.ObjenesisBase.newInstance(ObjenesisBase.java:73)
  at org.mockito.internal.creation.instance.ObjenesisInstantiator.newInstance(ObjenesisInstantiator.java:19)
  at org.mockito.internal.creation.bytebuddy.SubclassByteBuddyMockMaker.createMock(SubclassByteBuddyMockMaker.java:47)
  at org.mockito.internal.creation.bytebuddy.ByteBuddyMockMaker.createMock(ByteBuddyMockMaker.java:25)
  at org.mockito.internal.util.MockUtil.createMock(MockUtil.java:35)
  at org.mockito.internal.MockitoCore.mock(MockitoCore.java:69)
```

## How was this patch tested?

Pass the Jenkins with Maven build

Closes apache#23463 from dongjoon-hyun/SPARK-26536-2.

Authored-by: Dongjoon Hyun <[email protected]>
Signed-off-by: Dongjoon Hyun <[email protected]>

Author: dongjoon-hyun

sql/core/src/main/java/org/apache/spark/sql/sources/v2/reader/streaming/StreamingReadSupport.java

@@ -17,14 +17,17 @@
 
 package org.apache.spark.sql.sources.v2.reader.streaming;
 
+import com.google.common.annotations.VisibleForTesting;
+
 import org.apache.spark.sql.sources.v2.reader.ReadSupport;
 
 /**
- * A base interface for streaming read support. This is package private and is invisible to data
- * sources. Data sources should implement concrete streaming read support interfaces:
- * {@link MicroBatchReadSupport} or {@link ContinuousReadSupport}.
+ * A base interface for streaming read support. Data sources should implement concrete streaming
+ * read support interfaces: {@link MicroBatchReadSupport} or {@link ContinuousReadSupport}.
+ * This is exposed for a testing purpose.
  */
-interface StreamingReadSupport extends ReadSupport {
+@VisibleForTesting
+public interface StreamingReadSupport extends ReadSupport {
 
   /**
    * Returns the initial offset for a streaming query to start reading from. Note that the