SECURITY: Support basic authentication for webhook endpoints

[NickLarsenNZ]

Problem

There is no authentication on the webhook, meaning when the server is listening on 0.0.0.0, it is accessible from within the Kubernetes cluster (and possibly outside of the cluster depending on the CNI).

Proposal

• Support basic-auth for webhook endpoints.
• Add an example of how to use to the README.

See also #73