Shared: Add neutralModel extensible predicate

The neutralModel extensible predicate already exists in Java and C#, so
this change brings the dynamic languages more in line with static
languages. The Model Editor uses this predicate to mark endpoints as
"not interesting" from a data flow perspective.

python/ql/lib/semmle/python/frameworks/data/internal/ApiGraphModelsExtensions.qll

@@ -17,7 +17,7 @@ extensible predicate sourceModel(string type, string path, string kind);
 extensible predicate sinkModel(string type, string path, string kind);
 
 /**
- * Holds if calls to `(type, path)`, the value referred to by `input`
+ * Holds if in calls to `(type, path)`, the value referred to by `input`
  * can flow to the value referred to by `output`.
  *
  * `kind` should be either `value` or `taint`, for value-preserving or taint-preserving steps,
@@ -25,6 +25,13 @@ extensible predicate sinkModel(string type, string path, string kind);
  */
 extensible predicate summaryModel(string type, string path, string input, string output, string kind);
 
+/**
+ * Holds if calls to `(type, path)` should be considered neutral. The meaning of this depends on the `kind`.
+ * If `kind` is `summary`, the call does not propagate data flow. If `kind` is `source`, the call is not a source.
+ * If `kind` is `sink`, the call is not a sink.
+ */
+extensible predicate neutralModel(string type, string path, string kind);
+
 /**
  * Holds if `(type2, path)` should be seen as an instance of `type1`.
  */

python/ql/lib/semmle/python/frameworks/data/internal/empty.model.yml

@@ -15,6 +15,11 @@ extensions:
       extensible: summaryModel
     data: []
 
+  - addsTo:
+      pack: codeql/python-all
+      extensible: neutralModel
+    data: []
+
   - addsTo:
       pack: codeql/python-all
       extensible: typeModel