feat(providers): add Feishu (飞书/Lark) notification provider

[ayoubil]

Summary

Adds Feishu (飞书/Lark) notification provider for sending alerts to Feishu group chats via webhook.

Closes #6394

What is included

• keep/providers/feishu_provider/feishu_provider.py — Feishu webhook provider with optional sign verification
• docs/providers/documentation/feishu-provider.mdx — documentation
• mint.json navigation entry

Provider details

• Type: Notification (outbound)
• Auth: Webhook URL + optional signing secret
• Supports: Text and rich text (post) message types
• Security: Optional HMAC-SHA256 sign verification
• API: https://open.feishu.cn/document/client-docs/bot-v3/add-custom-bot

Same webhook pattern as Discord/Slack/Telegram providers.

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