Add getJsonBody util to plugin-sdk and document it

[0xtsukino]

Add getJsonBody util to plugin-sdk and document it

relates to #239

---

Decoding Raw JSON Body
For JSON POST requests, the body is in raw[].bytes as a number array:

function getJsonBody(request) {
  if (!request.requestBody?.raw?.[0]?.bytes) return null;

  // Convert number array to string
  const bytes = request.requestBody.raw[0].bytes;
  const text = String.fromCharCode(...bytes);

  try {
    return JSON.parse(text);
  } catch {
    return text;
  }
}

function main() {
  const requests = useRequests({ method: 'POST' });

  const targetRequest = requests.find(r => r.url.includes('/api/endpoint'));
  if (targetRequest) {
    const body = getJsonBody(targetRequest);
    // Use body data...
  }
}