This pull request introduces a new retriever called `rescorer`, which leverages the `rescore` functionality of the search request.
The `rescorer` retriever re-scores only the top documents retrieved by its child retriever, offering fine-tuned scoring capabilities.
All rescorers supported in the `rescore` section of a search request are available in this retriever, and the same format is used to define the rescore configuration.
<details>
<summary>Example:</summary>
```yaml
- do:
search:
index: test
body:
retriever:
rescorer:
rescore:
window_size: 10
query:
rescore_query:
rank_feature:
field: "features.second_stage"
linear: { }
query_weight: 0
retriever:
standard:
query:
rank_feature:
field: "features.first_stage"
linear: { }
size: 2
```
</details>
Closes#118327
Co-authored-by: Liam Thompson <32779855+leemthompo@users.noreply.github.com>
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