chore: add gitleaks config to allowlist testdata fixtures

Allowlist testdata/*.tf.tmpl paths in gitleaks config. These files
contain dummy private keys for acceptance tests, not real secrets.

Author: KT-Doan

.gitleaks.toml

@@ -0,0 +1,8 @@
+[extend]
+# Use the default gitleaks config as base
+useDefault = true
+
+[allowlist]
+paths = [
+  '''testdata/.*\.tf\.tmpl$''',
+]