Add capability for bwc

Author: elasticsearchmachine

x-pack/plugin/esql/qa/testFixtures/src/main/resources/knn-function.csv-spec

@@ -226,6 +226,7 @@ azure       | [240.0, 255.0, 255.0]
 
 knnWithNonPushableConjunction
 required_capability: knn_function_v5
+required_capability: knn_function_options_k_visit_percentage
 
 from colors metadata _score
 | eval composed_name = locate(color, " ") > 0 

x-pack/plugin/esql/src/main/java/org/elasticsearch/xpack/esql/action/EsqlCapabilities.java

@@ -1692,6 +1692,10 @@ public enum Cap {
          */
         TIME_SERIES_WINDOW_V0,
 
+        /**
+         * KNN function adds support for k and visit_percentage options
+         */
+        KNN_FUNCTION_OPTIONS_K_VISIT_PERCENTAGE,
         // Last capability should still have a comma for fewer merge conflicts when adding new ones :)
         // This comment prevents the semicolon from being on the previous capability when Spotless formats the file.
         ;