[HUDI-69] Support Spark Datasource for MOR table - RDD approach (#1848)

- This PR implements Spark Datasource for MOR table in the RDD approach.
- Implemented SnapshotRelation
- Implemented HudiMergeOnReadRDD
- Implemented separate Iterator to handle merge and unmerge record reader.
- Added TestMORDataSource to verify this feature.
- Clean up test file name, add tests for mixed query type tests
 - We can now revert the change made in DefaultSource

Co-authored-by: Vinoth Chandar <vchandar@confluent.io>

hudi-common/src/test/java/org/apache/hudi/common/testutils/HoodieTestDataGenerator.java

@@ -634,6 +634,10 @@ public class HoodieTestDataGenerator {
     return generateUniqueUpdatesStream(instantTime, n, TRIP_EXAMPLE_SCHEMA).collect(Collectors.toList());
   }
 
+  public List<HoodieRecord> generateUniqueUpdatesAsPerSchema(String instantTime, Integer n, String schemaStr) {
+    return generateUniqueUpdatesStream(instantTime, n, schemaStr).collect(Collectors.toList());
+  }
+
   /**
    * Generates deduped delete of keys previously inserted, randomly distributed across the keys above.
    *
@@ -745,6 +749,17 @@ public class HoodieTestDataGenerator {
     return result.stream();
   }
 
+  /**
+   * Generates deduped delete records previously inserted, randomly distributed across the keys above.
+   *
+   * @param instantTime Commit Timestamp
+   * @param n          Number of unique records
+   * @return List of hoodie records for delete
+   */
+  public List<HoodieRecord> generateUniqueDeleteRecords(String instantTime, Integer n) {
+    return generateUniqueDeleteRecordStream(instantTime, n).collect(Collectors.toList());
+  }
+
   public boolean deleteExistingKeyIfPresent(HoodieKey key) {
     Map<Integer, KeyPartition> existingKeys = existingKeysBySchema.get(TRIP_EXAMPLE_SCHEMA);
     Integer numExistingKeys = numKeysBySchema.get(TRIP_EXAMPLE_SCHEMA);