fix(terraform): convert input variables to expected type

[nikpivkin]

See aquasecurity/trivy#4997

The problem was that defsec did not convert variables to the expected type, which prevented variables of type set, map or object from being used as an argument to for_each. This PR adds conversion of input variable values to the expected type.

Also, defsec will now take into account the default values that the optional modifier sets if the value of a variable attribute is missing:

variable "policy_rules" {
  type = object({
    secure_tags = optional(map(object({
      session_matcher        = optional(string)
      priority               = number
      enabled                = optional(bool, true)
    })), {})
  })
}