ThreatModels: Add stdin kind

None of the current local subgroups precisely captures stdin, so
although it's much like both commandargs and file, a separate kind seems
better.

shared/mad/codeql/mad/ModelValidation.qll

@@ -119,6 +119,7 @@ module KindValidation<KindValidationConfigSig Config> {
         [
           // shared
           "local", "remote", "file", "commandargs", "database", "environment", "reverse-dns",
+          "stdin",
           // Java
           "android-external-storage-dir", "contentprovider",
           // C#

shared/threat-models/ext/threat-model-grouping.model.yml

@@ -15,6 +15,7 @@ extensions:
       - ["database", "local"]
       - ["commandargs", "local"]
       - ["environment", "local"]
+      - ["stdin", "local"]
       - ["file", "local"]
       - ["windows-registry", "local"]