Deprecate /_ltr/_stats endpoint and related legacy code

[rithin-pullela-aws]

Description

There are currently two endpoints providing the same LTR statistics functionality:

1. /_ltr/_stats
2. /_plugins/_ltr/stats

Additionally, all files in src/main/java/com/o19s/es/ltr/stats are marked for deprecation in 3.0.0 and are only being used in test cases.
Duplication for the same can be found in src/main/java/org/opensearch/ltr/stats

Current State

• /_ltr/_stats endpoint is not exposed.
• Duplicate functionality exists in /_plugins/_ltr/stats which is exposed.
• Legacy code in com.o19s.es.ltr.stats which is marked for deprecation is only used in test cases

Proposed Solution

Preferred Approach

1. Fully deprecate the /_ltr/_stats endpoint and related code
2. Standardize on /_plugins/_ltr/stats endpoint
3. Optionally maintain /_ltr/_stats as a legacy endpoint (can be added here)
4. Remove the deprecated folder (com.o19s.es.ltr.stats)
5. Update test cases to use the new implementation in org.opensearch.ltr.stats

Alternative Approach

1. Expose /_ltr/_stats endpoint
2. Remove deprecated folder
3. Update implementation accordingly

Questions to Consider

1. Are there any backward compatibility concerns?
2. Is anyone using the /_ltr/_stats endpoint, if we decide to deprecate it, should we provide a deprecation notice period?
3. Are there any external dependencies on the deprecated endpoint and the folder (com.o19s.es.ltr.stats)?

[sandeshkr419]

@JohannesDaniel Do you want to take this up? Thanks!