feat(notifications): webhook custom headers (#2230)

Signed-off-by: 0xsysr3ll <0xsysr3ll@pm.me>

server/lib/settings/index.ts

@@ -277,6 +277,7 @@ export interface NotificationAgentWebhook extends NotificationAgentConfig {
     webhookUrl: string;
     jsonPayload: string;
     authHeader?: string;
+    customHeaders?: { key: string; value: string }[];
     supportVariables?: boolean;
   };
 }