Add index.percolator.map_unmapped_fields_as_text index setting (#3850)

Relates: elastic/elasticsearch#28060

src/Nest/IndexModules/IndexSettings/Settings/FixedIndexSettings.cs

@@ -9,5 +9,12 @@ public static class FixedIndexSettings
 		public const string NumberOfRoutingShards = "index.number_of_routing_shards";
 		public const string NumberOfShards = "index.number_of_shards";
 		public const string RoutingPartitionSize = "index.routing_partition_size";
+
+		/// <summary>
+		/// If a field referred to in a percolator query does not exist,
+		/// it will be handled as a default text field so that adding the percolator query doesn't fail.
+		/// Defaults to <c>false</c>
+		/// </summary>
+		public const string PercolatorMapUnmappedFieldsAsText = "index.percolator.map_unmapped_fields_as_text";
 	}
 }