[SPARK-17627] Mark Streaming Providers Experimental

All of structured streaming is experimental in its first release.  We missed the annotation on two of the APIs.

Author: Michael Armbrust <michael@databricks.com>

Closes #15188 from marmbrus/experimentalApi.

sql/core/src/main/scala/org/apache/spark/sql/sources/interfaces.scala

@@ -112,8 +112,10 @@ trait SchemaRelationProvider {
 }
 
 /**
+ * ::Experimental::
  * Implemented by objects that can produce a streaming [[Source]] for a specific format or system.
  */
+@Experimental
 trait StreamSourceProvider {
 
   /** Returns the name and schema of the source that can be used to continually read data. */
@@ -132,8 +134,10 @@ trait StreamSourceProvider {
 }
 
 /**
+ * ::Experimental::
  * Implemented by objects that can produce a streaming [[Sink]] for a specific format or system.
  */
+@Experimental
 trait StreamSinkProvider {
   def createSink(
       sqlContext: SQLContext,