Add dedicated alias type for CRD array item enums

[rainest]

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Problem Statement

Follow-up from #1757 (comment)

Per kubernetes-sigs/controller-tools#342 our tooling does not support automated validation generation for arrays of standard types where we wish to apply validation rules to the items within the array. For example, if you wish to enforce that every string item in the protocols array is one of http, https, udp, or tcp, you cannot.

Proposed Solution

Absent functionality in controller-tools (which is unlikely to arrive any time soon), the prescribed solution is to create a dedicated alias type. You can write something like:

//+kubebuilder:validation:Enum=http;https;grpc;grpcs;tcp;tls;udp
type KongPluginProtocol string

Protocols []KongPluginProtocol `json:"protocols,omitempty"`

where we've currently tried

//+kubebuilder:validation:Enum=http;https;grpc;grpcs;tcp;tls;udp
Protocols []string `json:"protocols,omitempty"`

and Kubebuilder will build the correct validation rule.

Additional information

No response

Acceptance Criteria

• All arrays in our CRDs that need validation rules applied to their items have dedicated types with validation annotations.
• The generated CRDs have the validation rules we want (for the most part, validation rules that match the hand-written CRDs in https://github.com/Kong/kubernetes-ingress-controller/blob/1.3.1/deploy/manifests/base/custom-types.yaml).