[terraform] Fix kubernetes_resources defaults

[bernardjkim]

This PR implements a plan_modifier for role.spec.allow.kubernetes_resources to handle default value changes across role versions.

Without this modifier, users have had trouble upgrading between role versions using the Terraform provider. The new plan modifier ensures that defaults for kubernetes_resources are recomputed during version upgrades.

For example, v7 roles provide the following default values for kubernetes_resources:

kubernetes_resources:
- kind: '*'
  name: '*'
  namespace: '*'
  verbs:
  - '*'

The v8 role now requires api_group and uses the following default:

kubernetes_resources:
- api_group: '*'
  kind: '*'
  name: '*'
  namespace: '*'
  verbs:
  - '*'

Without this new plan modifier, users will not be able to upgrade from v7 to v8 without manually specifying the kubernetes_resources.api_group within the Terraform config. This change should also fix previous issues with even older role versions.

Changelog: Added a plan modifier to recompute kubernetes_resources defaults during role version upgrades, fixing Terraform role upgrade issues.

[backport-bot-workflows]

@bernardjkim See the table below for backport results.

Branch	Result
branch/v18	Create PR