Add support for isDefined in log template

Add isDefined as top level predicate for log template values

Author: jpbempel

dd-java-agent/agent-debugger/debugger-el/src/main/java/com/datadog/debugger/el/JsonToExpressionConverter.java

@@ -50,7 +50,8 @@ public class JsonToExpressionConverter {
               "endsWith",
               "contains",
               "matches",
-              "instanceof"));
+              "instanceof",
+              "isDefined"));
 
   @FunctionalInterface
   interface BinaryPredicateExpressionFunction<T extends Expression> {

dd-java-agent/agent-debugger/debugger-el/src/test/resources/test_one_liner_value_expr_01.txt

@@ -19,3 +19,4 @@
 {"dsl": "", "json": {"contains": [{"ref": "str"}, "ll"]}}
 {"dsl": "", "json": {"matches": [{"ref": "str"}, "[helo]+"]}}
 {"dsl": "", "json": {"instanceof": [{"ref": "str"}, "java.lang.String"]}}
+{"dsl": "", "json": {"isDefined": {"ref": "str"}}}

dd-java-agent/agent-debugger/debugger-el/src/test/resources/test_value_expr_01.json

@@ -24,7 +24,8 @@
           {"endsWith": [{"ref": "str"}, "llo"]},
           {"contains": [{"ref": "str"}, "ll"]},
           {"matches": [{"ref": "str"}, "[helo]+"]},
-          {"instanceof": [{"ref": "str"}, "java.lang.String"]}
+          {"instanceof": [{"ref": "str"}, "java.lang.String"]},
+          {"isDefined": {"ref": "str"}}
         ]
       }
     ]