[HUDI-2593] Virtual keys support for metadata table (#3968)

- Metadata table today has virtual keys disabled, thereby populating the metafields
  for each record written out and increasing the overall storage space used. Hereby
  adding virtual keys support for metadata table so that metafields are disabled
  for metadata table records.

- Adding a custom KeyGenerator for Metadata table so as to not rely on the
  default Base/SimpleKeyGenerators which currently look for record key
  and partition field set in the table config.

- AbstractHoodieLogRecordReader's version of processing next data block and
  createHoodieRecord() will be a generic version and making the derived class
  HoodieMetadataMergedLogRecordReader take care of the special creation of
  records from explictly passed in partition names.

hudi-common/src/test/java/org/apache/hudi/common/functional/TestHoodieLogFormat.java

@@ -1672,9 +1672,9 @@ public class TestHoodieLogFormat extends HoodieCommonTestHarness {
                                        Map<HeaderMetadataType, String> header) {
     switch (dataBlockType) {
       case AVRO_DATA_BLOCK:
-        return new HoodieAvroDataBlock(records, header);
+        return new HoodieAvroDataBlock(records, header, HoodieRecord.RECORD_KEY_METADATA_FIELD);
       case HFILE_DATA_BLOCK:
-        return new HoodieHFileDataBlock(records, header);
+        return new HoodieHFileDataBlock(records, header, HoodieRecord.RECORD_KEY_METADATA_FIELD);
       default:
         throw new RuntimeException("Unknown data block type " + dataBlockType);
     }