Custom webhook support

[ibrahimcesar]

Description

Send telemetry events to custom webhook endpoints for integration with any system.

Features

• Configurable webhook URLs
• Event filtering (which events to send)
• Payload transformation templates
• Retry with exponential backoff
• Signature verification (HMAC)

Example Usage

```rust
TelemetryKit::builder()
.service_name("my-app")
.webhook(WebhookConfig {
url: "https://example.com/webhook",
events: vec!["command.executed", "feature.used"],
secret: env::var("WEBHOOK_SECRET")?,
})
.build()?;
```

Use Cases

• Slack notifications
• Discord alerts
• Custom analytics pipelines
• IFTTT/Zapier integration

Acceptance Criteria

• Reliable delivery with retries
• Configurable payload format
• Secure signature verification