support users setting their own CA for webhook receiver <-> kube api-server communication

[stuartnelson3]

Currently, the helm chart is generating a self-signed cert for securing communication on install:

https://github.com/elastic/apm-mutating-webhook/blob/main/apm-agent-auto-attach/templates/webhook.yaml#L16-L20

Users may want to bring their own certs for communication; look into supporting this.

[axw]

Repeating a comment I made on Slack for posterity: I would push back on this unless there's a strong reason to BYO certs. What's implemented is nicely automated by Helm, and is secure.