[RFC-52][HUDI-3907] RFC for Introduce Secondary Index to Improve Hudi Query Performance

[huberylee]

What is the purpose of the pull request

RFC for Introduce Secondary Index to Improve HUDI Query Performance

Brief change log

• Modify rfc/README.md
• Add rfc/rfc-52 dir

Verify this pull request

(Please pick either of the following options)

This pull request is a trivial rework / code cleanup without any test coverage.

(or)

This pull request is already covered by existing tests, such as (please describe tests).

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This change added tests and can be verified as follows:

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• Added integration tests for end-to-end.
• Added HoodieClientWriteTest to verify the change.
• Manually verified the change by running a job locally.

Committer checklist

• Has a corresponding JIRA in PR title & commit

• Commit message is descriptive of the change

• CI is green

• Necessary doc changes done or have another open PR

• For large changes, please consider breaking it into sub-tasks under an umbrella JIRA.

[huberylee]

The previous pull request was accidentally closed, some discussions can be found there: #5370

[hudi-bot]

CI report:

• f8863fd Azure: PENDING

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