Add Semgrep rule for detecting base64-encoded string literals in Go code

.semgrep/malicious-api.yml

@@ -284,3 +284,17 @@ rules:
     confidence: low
     likelihood: medium
     impact: high
+
+- id: go-base64-literal-string
+  languages: [go]
+  message: "Base64-encoded string literal detected. Review for embedded payloads/secrets."
+  severity: INFO
+  pattern: "\"$B64\""
+  metavariable-regex:
+    metavariable: $B64
+    regex: "^(?:[A-Za-z0-9+/]{16,}={0,2}|[A-Za-z0-9_-]{16,}={0,2})$"
+  metadata:
+    category: security
+    confidence: low
+    likelihood: low
+    impact: medium