Add variable validation for customer_id

[xingao267]

Something like the following

variable "customer_id" {
  description = "..."
  type        = string
  validation {
    condition     = can(regex("^customers/C0[a-z0-9]{7}$", var.customer_id))
    error_message = "Customer ID must be in the form customers/C0xxxxxxx."
  }
}