Add Support for Generating a Default Wildcard Certificate

[danehans]

What would you like to be added:
The ability for Gateway to support generating a wildcard certificate.

Why is this needed:
To simplify the user experience, it's beneficial for Gateway to generate a default wildcard certificate for use by a listener. If the listener specifies one or more certificates, those certificates are used for authenticating requests and SNI-based routing. If the listener does not specify any certificates, then a wildcard certificate is generated and used by the listener to authenticate the request and perform SNI-based routing. A couple approaches are presented here for generating the wildcard certificate subdomain:

1. Constructing the subdomain from the listener's Name. For example:

kind: Gateway
spec:
  listeners: 
  - name: foo.example.com
     protocol: HTTPS
     # no TLS config

The above spec would cause the Gateway to generate a wildcard certificate for *.example.com.

2. Adding a Domain field to Gateway as specified in Add Domain Support #62.

kind: Gateway
spec:
  domain: example.com
  listeners: 
  - name: foo.example.com
     protocol: HTTPS
     # no TLS config

The above spec would cause the Gateway to generate a wildcard certificate for *.example.com.

/assign @bowei
/cc @jpeach @ironcladlou @Miciah

[danehans]

xref: #49

[danehans]

Closing as generating a default wildcard certificate is not supported by the general ingress controller community. Generating a wildcard certificate for use by a gateway listener should be explicit and not generated by default.