Introduce STS client grants API and OPA policy integration (#6168)

This PR introduces two new features

- AWS STS compatible STS API named AssumeRoleWithClientGrants

```
POST /?Action=AssumeRoleWithClientGrants&Token=<jwt>
```

This API endpoint returns temporary access credentials, access
tokens signature types supported by this API

  - RSA keys
  - ECDSA keys

Fetches the required public key from the JWKS endpoints, provides
them as rsa or ecdsa public keys.

- External policy engine support, in this case OPA policy engine

- Credentials are stored on disks

cmd/bucket-notification-handlers.go

@@ -139,7 +139,7 @@ func (api objectAPIHandlers) PutBucketNotificationHandler(w http.ResponseWriter,
 		return
 	}
 
-	if err = saveNotificationConfig(objectAPI, bucketName, config); err != nil {
+	if err = saveNotificationConfig(ctx, objectAPI, bucketName, config); err != nil {
 		writeErrorResponse(w, toAPIErrorCode(err), r.URL)
 		return
 	}