Add optional TLS support for the operator metrics server (port 8080) to
enhance security when exposing Prometheus metrics.

The feature is opt-in and controlled entirely by the METRICS_CERT_DIR
environment variable. When set, the operator will:
- Enable TLS (SecureServing) for the metrics server
- Look for certificates at the specified path
- Use standard Kubernetes TLS secret naming (tls.crt/tls.key)

When METRICS_CERT_DIR is not set (default), the metrics server continues
to operate without TLS, ensuring no breaking changes to existing deployments.

Usage:
1. Create a Kubernetes secret with TLS certificates (tls.crt and tls.key)
2. Mount the secret to the operator pod (e.g., /run/secrets/cnpg.io/metrics)
3. Set METRICS_CERT_DIR environment variable to the mount path
4. Update monitoring configuration (e.g., PodMonitor) to use HTTPS

Signed-off-by: Armando Ruocco <armando.ruocco@enterprisedb.com>

Signed-off-by: Niccolò Fei <niccolo.fei@enterprisedb.com>

Signed-off-by: Niccolò Fei <niccolo.fei@enterprisedb.com>

Signed-off-by: Marco Nenciarini <marco.nenciarini@enterprisedb.com>

Signed-off-by: Gabriele Bartolini <gabriele.bartolini@enterprisedb.com>