Revisit setRecordMerger API in HoodieReaderContext

[https://github.com/apache/hudi/pull/11943#discussion_r1805545250]

The HoodieReaderContext has the following API which takes `Option`.  By design, the merge implementation class should always be non-empty.  So we need to see if changing the argument type to `HoodieRecordMerger` incurs any error.
{code:java}
public void setRecordMerger(Option recordMerger) {
this.recordMerger = recordMerger;
}{code}

## JIRA info

- Link: https://issues.apache.org/jira/browse/HUDI-8417
- Type: Improvement
- Fix version(s):
- 1.1.0