Revise CRD code markers/validations

[pmalek]

Problem statement

#5103 tries to solve #5061 but leaves the CRD validations unreviewed.

For instance:

• KongConsumer's Username is required (enforced in admission webhook) but that could easily be changed to a required field on the CRD level via:

      // +kubebuilder:validation:Required
      Username string `json:"username"`
  

  This unfortunately can only be done when the // +kubebuilder:validation:Optional is removed from the package as that overrides the field level setting.

Acceptance criteria

• All Kong provided CRDs are reviewed for moving validation code from admission webhook to CRD level and all possible changes are merged to main