Allow configuration of an external webhook and its associated certs in the Porch API server

[liamfallon]

The Porch API server can use a validating webook to check whether delete requests should be allowed or not. If the CERT_STORAGE_DIR environment variable is not set, Porch does not validate deletions. If the CERT_STORAGE_DIR environment variable is set, Porch generates a validating webhook and its associated certs.

Using a webhook ensures that resources are only deleted when the user has authority to do so.

However, the Porch API server cannot today be configured to use an externally configured webhook. In addition, the current self generated webhook has a number of drawbacks:

• Access to the deletion commands cannot be controlled by configuration
• The generated certificates time out after a year
• The service name on which the webhook acts and the configuratino name cannot be controlled by configuration
• The namespace on which Porch is installed is hardcoded to "porch-system" {fixed in PR-26}

This issue proposes adding support for externally configured webhooks to the Porch API server.

[liamfallon]

/assign @Catalin-Stratulat-Ericsson

[liamfallon]

Triaged

[efiacor]

nephio-project/porch#53