feat(notification): add task status notification module with DingTalk support

[p-yf]

Describe what this PR does / why we need it

Add a new notification module that provides an extensible NotifierService interface, allowing developers to implement custom notification channels (e.g., DingTalk, Feishu, WeCom). A default DingTalk implementation DingTalkNotifier is provided with HmacSHA256 signature verification.

Agent tasks can take a long time to execute. When user intervention is required, without a notification mechanism users cannot be notified in time, potentially causing task delays.

Describe how you did it

1. Added core interface NotifierService defining notification methods
2. Added NotifierFactory to support fetching notifiers by name or getting the default
3. Added NotificationInfo containing node name, status, timestamp, etc.
4. Provided DingTalkNotifier implementation with Webhook + signature authentication

Describe how to verify it

1. Configure DingTalk robot Webhook URL and Secret Key: spring.ai.alibaba.data-agent.notify.channel: dingtalk
   spring.ai.alibaba.data-agent.notify.dingtalk.webhook-url: https://oapi.dingtalk.com/robot/send?access_token=xxx
   spring.ai.alibaba.data-agent.notify.dingtalk.secret-key: SECxxx
2. Run NotificationInfoTest and DingTalkNotifierTest unit tests
3. Integrate into business nodes, call NotifierFactory.getDefault().notify(info) to send notifications

Special notes for reviews

• Notifiers are auto-registered via @component, developers can implement NotifierService and add @component annotation to extend other channels
• DingTalkNotifier uses @value to inject configuration. The notify.dingtalk.webhook-url must be configured for messages to be sent; if not configured, notification is skipped