[Workload Management] Add rule schema for workload management

[ruai0511]

Description

This PR introduces the schema for Rule object used in the workload management feature

{
    "_id": "fwhbuib397u4o03=="
    "index_pattern": ["logs123", "user*"],
    "query_group": "dev_query_group_id",
    "updated_at": "01-10-2025T21:23:456Z"
}

Related issues:
RFC: #16797
#16813

Check List

• Functionality includes testing.
• API changes companion pull request created, if applicable.
• Public documentation issue/PR created, if applicable.

[github-actions]

✅ Gradle check result for f6a4a28: SUCCESS

[codecov]

Codecov Report

Attention: Patch coverage is 82.60870% with 24 lines in your changes missing coverage. Please review.

Project coverage is 72.44%. Comparing base (faabd10) to head (bb8d06c).
Report is 19 commits behind head on main.

Files with missing lines	Patch %	Lines
server/src/main/java/org/opensearch/wlm/Rule.java	82.60%	9 Missing and 15 partials ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##               main   #17238      +/-   ##
============================================
+ Coverage     72.36%   72.44%   +0.07%     
- Complexity    65733    65825      +92     
============================================
  Files          5318     5319       +1     
  Lines        305674   305812     +138     
  Branches      44349    44376      +27     
============================================
+ Hits         221197   221535     +338     
+ Misses        66341    66128     -213     
- Partials      18136    18149      +13     

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[github-actions]

❕ Gradle check result for bb8d06c: UNSTABLE

Please review all flaky tests that succeeded after retry and create an issue if one does not already exist to track the flaky failure.