Adding support for mTLS authentication to the argo-rollouts Notifications Engine’s HTTP transport layer

### Summary
This issue is for adding support for mutual TLS (mTLS) authentication to the argo-rollouts Notifications Engine’s HTTP transport layer.
The feature enables webhook integrations to perform secure, bi-directional authentication between the client and server using certificates.
### Motivation
Currently, the argo-rollouts Notifications Engine supports webhook authentication using standard mechanisms (e.g., Basic Auth, bearer tokens).
However, for enterprise or compliance-critical environments, mTLS is required.
This enhancement will expand support for secure webhook communication, particularly in regulated or internal environments.
### Proposal
A new function will be added to create an `http.Transport` configured for mTLS authentication.
It will load client and CA certificates from the provided configuration, build a corresponding `tls.Config`, and return a transport that enables secure, bi-directional authentication between the Notifications Engine and webhook endpoints.
This ensures encrypted and mutually trusted communication for webhook notifications.

Sample configMap illustrating how to configure a webhook that requires mutual TLS authentication:

```
apiVersion: v1
kind: ConfigMap
metadata:
name: argocd-notifications-cm
data:
service.webhook.secure-api: |
url: https://secure-api.company.com/webhooks/notifications
headers:
- name: Content-Type
value: application/json
- name: X-API-Version
value: v1
mtlsConfig:
certPath: /etc/ssl/certs/webhook-client.crt
keyPath: /etc/ssl/private/webhook-client.key
caCertPath: /etc/ssl/certs/company-ca.crt
insecureSkipVerify: false # Enforce proper certificate validation
template.secure-notification: |
webhook:
secure-api:
method: POST
body: |
{
"event": "application-sync",
"application": "{{.app.metadata.name}}",
"namespace": "{{.app.metadata.namespace}}",
"status": "{{.app.status.sync.status}}",
"revision": "{{.app.status.sync.revision}}",
"timestamp": "{{.time.Now}}",
"argocd_url": "{{.context.argocdUrl}}/applications/{{.app.metadata.name}}"
}
```