Make sure Hudi Spark relations implementations provide similar file-scanning metrics

After rebasing Hudi's COW table onto `BaseFileOnlyRelation` from `HadoopFsRelation`, we lost very helpful file-scanning metrics like # of files scanned, total size of files scanned, etc.

After some investigation, i've found out that this occurred b/c now instead of relying on `FileScan` node in Spark plans like we're before, it now uses `DataScan` node that doesn't provide such metrics.

This unwanted transition occurred b/c Spark internally predicates on `HadoopFsRelation` to decide whether it's a `FileScan` or `DataScan`, and since we stopped using `HadoopFsRelation` Hudi relations now fall into the latter bucket

## JIRA info

- Link: https://issues.apache.org/jira/browse/HUDI-3882
- Type: Improvement
- Epic: https://issues.apache.org/jira/browse/HUDI-1297

Start by tracing Hudi's COW table relation implementation and how Spark chooses between FileScan and DataScan for HadoopFsRelation. Compare the available file-scanning metrics and determine how Hudi relations can provide equivalent metrics; done means plans expose metrics such as files scanned and total scanned-file size again.

Written by the indexing model from the issue text.