[HUDI-1663] Streaming read for Flink MOR table (#2640)

Supports two read modes:
* Read the full data set starting from the latest commit instant and
  subsequent incremental data set
* Read data set that starts from a specified commit instant

hudi-common/src/main/java/org/apache/hudi/common/model/HoodieCommitMetadata.java

@@ -36,6 +36,7 @@ import java.io.Serializable;
 import java.nio.charset.StandardCharsets;
 import java.util.ArrayList;
 import java.util.HashMap;
+import java.util.HashSet;
 import java.util.List;
 import java.util.Map;
 
@@ -339,6 +340,10 @@ public class HoodieCommitMetadata implements Serializable {
         maxEventTime == Long.MIN_VALUE ? Option.empty() : Option.of(maxEventTime));
   }
 
+  public HashSet<String> getWritePartitionPaths() {
+    return new HashSet<>(partitionToWriteStats.keySet());
+  }
+
   @Override
   public boolean equals(Object o) {
     if (this == o) {