ci: Add Bandit security linter to CI (#252)

Introduce Bandit to CI checks.

Fixes: #251 🦕

Author: lukehinds

.github/workflows/security.yaml

@@ -0,0 +1,19 @@
+name: Bandit
+
+on:
+  workflow_dispatch:
+
+jobs:
+  analyze:
+    runs-on: ubuntu-latest
+    permissions:
+      security-events: write
+      actions: read
+      contents: read
+    steps:
+      - name: Perform Bandit Analysis
+        uses: PyCQA/bandit-action@v1
+        with:
+          severity: medium
+          confidence: medium
+          targets: "src/a2a"