Add l2_norm normalization support to linear retriever (elastic#128504)

* New l2 normalizer added

* L2 score normaliser is registered

* test case added to the yaml

* Documentation added

* Resolved checkstyle issues

* Update docs/changelog/128504.yaml

* Update docs/reference/elasticsearch/rest-apis/retrievers.md

Co-authored-by: Copilot <[email protected]>

* Score 0 test case added to check for corner cases

* Edited the markdown doc description

* Pruned the comment

* Renamed the variable

* Added comment to the class

* Unit tests added

* Spotless and checkstyle fixed

* Fixed build failure

* Fixed the forbidden test

---------

Co-authored-by: Copilot <[email protected]>

Author: mridula-s109

docs/changelog/128504.yaml

@@ -0,0 +1,5 @@
+pr: 128504
+summary: Add l2_norm normalization support to linear retriever
+area: Relevance
+type: enhancement
+issues: []