Introduce the SourceNode and ThreatModelFlowSource classes

1. Introduces the `SourceNode` class which allows dataflow nodes
   representing sources to indicate the threat model they are associated
   with.
2. Introduces the `ThreatModelFlowSource` class which represents a
   source node which respects the threat model configuration

csharp/ql/lib/semmle/code/csharp/security/dataflow/flowsources/SourceNode.qll

@@ -0,0 +1,26 @@
+private import semmle.code.csharp.dataflow.internal.ExternalFlow
+private import codeql.threatmodels.ThreatModels
+
+/**
+ * A data flow source.
+ */
+abstract class SourceNode extends DataFlow::Node {
+  /**
+   * Gets a string that represents the source kind with respect to threat modeling.
+   */
+  abstract string getThreatModel();
+}
+
+/**
+ * A class of data flow sources that respects the
+ * current threat model configuration.
+ */
+class ThreatModelFlowSource extends DataFlow::Node {
+  ThreatModelFlowSource() {
+    exists(string kind |
+      // Specific threat model.
+      currentThreatModel(kind) and
+      (this.(SourceNode).getThreatModel() = kind or sourceNode(this, kind))
+    )
+  }
+}