Add TOP_SNIPPETS function to return the best snippets for a field (elastic#138940)

* Add TOP_SNIPPETS function to return the best snippets for a field

* [CI] Auto commit changes from spotless

* Exclude top snippets from CSV tests

* Update docs/changelog/138940.yaml

* num words and num snippets must be positive integers

* PR feedback - create chunking settings and scorer in the evaluator

* Clarify query

* PR feedback - clean up tests

* Make top snippets work with CSV tests

* Add spec for null field

* [CI] Auto commit changes from spotless

* Add null input to verifier test

* Ensure verifier tests catch when num_words or num_snippets are not positive integers

* Checkstyle fix

---------

Co-authored-by: elasticsearchmachine <infra-root+elasticsearchmachine@elastic.co>

Author: kderusso

docs/changelog/138940.yaml

@@ -0,0 +1,5 @@
+pr: 138940
+summary: Add TOP_SNIPPETS function to return the best snippets for a field
+area: ES|QL
+type: enhancement
+issues: []