feat(eks): expose configurable variables documented in README

[Copilot]

The EKS module README documented variables like kubernetes_version, endpoint_public_access, endpoint_private_access, enable_cluster_creator_admin_permissions, and authentication_mode, but these were hardcoded in the module implementation rather than exposed as inputs.

Changes:

• Added 5 missing input variables to variables.tf with defaults matching the previously hardcoded values
• Updated main.tf to use variables instead of literals
• Maintains backward compatibility—existing configurations work unchanged

Before:

# main.tf - hardcoded values
kubernetes_version = "1.32"
endpoint_public_access  = false
endpoint_private_access = true

After:

# variables.tf - configurable with sensible defaults
variable "kubernetes_version" {
  description = "Kubernetes version for the EKS cluster"
  type        = string
  default     = "1.32"
}

variable "endpoint_public_access" {
  description = "Enable public API server endpoint access"
  type        = bool
  default     = false
}
# ... additional variables

# main.tf - uses variables
kubernetes_version = var.kubernetes_version
endpoint_public_access  = var.endpoint_public_access
endpoint_private_access = var.endpoint_private_access

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