Utilize CEL validation in Contour CRDs

[sunjayBhatia]

Our CRD field validation is a bit inconsistent and could use a refresh. We don't have consistent validation for similar fields and should probably introduce dedicated common types for some. For example, namespace name validation:

• contour/apis/projectcontour/v1/httpproxy.go

  Lines 50 to 51 in 9eb2838

  	// +optional
  	Namespace string `json:"namespace,omitempty"`

• contour/apis/projectcontour/v1/httpproxy.go

  Lines 209 to 211 in 9eb2838

  	// +optional
  	// +kubebuilder:validation:MinLength=1
  	Namespace string `json:"namespace,omitempty" protobuf:"bytes,2,opt,name=namespace"`

Also, much of our HTTPProxy|ExtensionService|etc. Processor logic is dedicated to validating CRD fields and setting Invalid status on resources that have bad config. If we can use CEL validations to do some of this work at resource creation time we may be able to prevent This would enable us to immediately surface configuration errors rather than requiring users to create a resource, check status, etc.

• HTTPProxy
• TLSCertificateDelegation
• ContourConfiguration
• ContourDeployment
• ExtensionService

[flawedmatrix]

Assigning to myself since I've been looking at this today.