Add reader state class to remove state from the reader context

Reader context is getting cluttered with state. Create new class to contain all that so we can keep the reader context a bit more simple

## JIRA info

- Link: https://issues.apache.org/jira/browse/HUDI-8170
- Type: Improvement
- Epic: https://issues.apache.org/jira/browse/HUDI-6243
- Fix version(s):
- 1.1.0

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## Comments

23/Oct/24 06:30;yihua;In HUDI-8203 (https://github.com/apache/hudi/pull/11943), record merger is made `Option` (see https://github.com/apache/hudi/pull/11943#discussion_r1805544464).  We should make it final so that there is no chance of hitting NPE or merge implementation class not set.;;;

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23/Oct/24 06:52;yihua;Another improvement: we should get rid of the call `readerContext.setRecordMerger(readerContext.getRecordMerger(...))` and only have `setRecordMerger` for simplicity (see details in https://github.com/apache/hudi/pull/11943#discussion_r1805660051).;;;