Enable the abiltiy to support nested and non-nested message descriptors

Protobuf definition files can have messages defined in a nested and non-nested form.

Nested:

package bigdata;
message DataEntity {
..
   optional MetaData context = 6;

  message MetaData {
 ... 
 }
}

Non-nested:

package bigdata;

message DataEntity {
..
   optional MetaData context = 6;
}
  message MetaData {
 ... 
 }

The ProtobufMetadataProcessor needs to be changed to so that it can create the metadata from either use case.

Currently it obtains the child descriptor messages from the parent descriptor.