feat: Add Discord and Generic Webhook notification providers

[kCn3333]

feat: Add Discord and Generic Webhook notification providers

Extends notification system with Discord rich embeds and universal webhook support.

Discord Features:

• Inline fields for structured data
• Custom bot username and avatar
• Automatic timestamps and footers
• Priority-based color coding (green/yellow/red)

Generic Webhook Features:

• Universal HTTP POST to any endpoint
• Custom headers via query params (header_*)
• JSON payload with title, message, priority
• Works with Zapier, Make.com, n8n, custom APIs
• Metadata support from query parameters

Enhanced:

• NotificationMessageBuilder with structured data extraction
• NotificationColor enum for consistent theming
• Extended documentation with provider examples
• URL format: discord://webhook-id/token
• URL format: webhook://domain/path?header_Key=Value

Backward compatible with existing NTFY configuration.