fix: make webhook cert Secret name configurable

[niyomukiza-mechack]

What was changed

Replaced the hard-coded webhook-server-cert Secret name with a new configurable Helm value webhook.certSecretName. Previously, the Secret name was hard coded in both templates/manager.yaml and templates/certmanager.yaml so users who bring their own TLS certificate had to name their Secret exactly webhook-server-cert no other name worked. Now they can set webhook.certSecretName to match whatever Secret they've created. The default is still webhook-server-cert so existing installations are unaffected.

Files changed:

values.yaml - added webhook.certSecretName with default value updated the caBundle comment to reference the value instead of the literal name
templates/manager.yaml - replaced hard coded secretName: webhook-server-cert with {{ .Values.webhook.certSecretName }}
templates/certmanager.yaml - same replacement
values.schema.json - added certSecretName property to the webhook schema

Why?

Context: #316

The webhook cert Secret name was hard coded as webhook-server-cert in two template files with no way to configure it. This caused a poor user experience: users who set certmanager.enabled: false and brought their own TLS certificate were forced to name their Secret exactly webhook-server-cert or the controller pod would get stuck in ContainerCreating forever. This is part of the larger webhook configuration cleanup tracked in #316.

Checklist

1. Closes

2. How was this tested:

Verified the new value flows through to both templates
Custom Secret name renders in both manager.yaml and certmanager.yaml and Default value preserves backward compatibility

3. Any docs updates needed?