ESQL: Add capability to prevent failing mixed tests after Starts/EndsWith Lucene pushdown (elastic#124764)

Continuation of elastic#124641, after different issues arised.

Closes elastic#124693
Closes elastic#124745

The failing BWC tests are checking <8.14 mixed cluster nodes, and SingleValueQuery is sometimes emitting warnings with an empty source.

Author: ivancea

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

@@ -991,6 +991,7 @@ beta         | Kubernetes cluster
 ;
 
 startsWithLucenePushdownIgnoreMultivalues
+required_capability: starts_with_ends_with_lucene_pushdown
 
 from hosts
 | where starts_with(description, "epsilon")
@@ -1259,6 +1260,7 @@ beta         | Kubernetes cluster
 ;
 
 endsWithLucenePushdownIgnoreMultivalues
+required_capability: starts_with_ends_with_lucene_pushdown
 
 from hosts
 | where ends_with(description, "host")

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

@@ -676,6 +676,12 @@ public enum Cap {
          */
         REMOVE_REDUNDANT_SORT,
 
+        /**
+         * Lucene query pushdown to StartsWith and EndsWith functions.
+         * This capability was created to avoid receiving wrong warnings from old nodes in mixed clusters
+         */
+        STARTS_WITH_ENDS_WITH_LUCENE_PUSHDOWN,
+
         /**
          * Allow mixed numeric types in conditional functions - case, greatest and least
          */