feat: add Feishu (飞书/Lark) notification provider (#6394)

[qiuluoli]

Implements issue #6394: Feishu (飞书/Lark) notification provider for Keep.

What

• Full Feishu bot webhook notification provider
• Follows the standard Keep provider pattern (BaseProvider, AuthConfig, etc.)
• Same structure as Slack/Discord/Telegram providers

Features

• Webhook-based bot notifications (Incoming Webhook)
• Optional sign verification (HMAC-SHA256) for security
• 3 message types: text, interactive card, rich post
• Severity-based color coding (critical=red, high=orange, warning=yellow, etc.)
• Alert data formatting with name, severity, description, source

Implementation

• FeishuProviderAuthConfig: webhook_url (required), webhook_secret (optional)
• FeishuProvider: validate_config, dispose, _notify, _generate_sign
• Provider category: Collaboration
• API docs: https://open.feishu.cn/document/client-docs/bot-v3/add-custom-bot

Testing

Can be tested by setting FEISHU_WEBHOOK_URL environment variable and running the provider directly.

/attempt #6394

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[shahargl]

N/A