title: How to decode json with unknown key
body: <p>everyone, how to serialize json struct, if one of field unknown?</p>
  
  <p>My json is:    </p>
  
  <pre><code> {"brands":{"any":false,"19":{"is_all":false,"values":[185,182,178],"include":true},"23":{"is_all":false,"values":[198,199,201],"include":true}},"price":[2000,10000],"year":[1990,2018],"fuel_type":[1,2],"engine_capacity":[\"1.8\",\"4.8\"],"color":[1,2,3],"gearbox_id":[2],"is_georgia":false}
  </code></pre>
  
  <p>but: </p>
  
  <p>"19":{"is_all":false,"values":[185,182,178],"include":true}</p>
  
  <p>"23":{"is_all":false,"values":[198,199,201],"include":true}</p>
  
  <p>19 and 23 - is string unknown value, it's generated by API. </p>
  
  <p>So my struct is:</p>
  
  <pre><code>    struct auto_order_model: Decodable {
          var brands: brand_details             &lt;---- this is problem
          let price: [Int]
          let year: [Int]
          let fuel_type: [Int]
          let engine_capacity: [String]
          let color: [Int]
          let gearbox_id: [Int]
          let is_georgia: Bool
      }
      struct brand_details: Decodable {
          var any: Bool
          var brand_num: [models]?
      }
      struct models: Decodable {
          var is_all: Bool
          var values: [Int]
          var include: Bool
      }
  </code></pre>
  
  <p>i decode this json like this:</p>
  
  <pre><code>    do {
          let data = try JSONDecoder().decode(auto_order_model.self, from: json)
          print(data)
      }catch {
          print("JSON Error")
      }
  </code></pre>
  
  <p>so, on output I get <strong>nil</strong> of brand_num:</p>
  
  <pre><code>▿ auto_order_model #1
    ▿ brands : brand_details #1
      - any : false
      - brand_num : nil
    ▿ price : 2 elements
      - 0 : 2000
      - 1 : 10000
    ▿ year : 2 elements
      - 0 : 1990
      - 1 : 2018
    ▿ fuel_type : 2 elements
      - 0 : 1
      - 1 : 2
    ▿ engine_capacity : 2 elements
      - 0 : "1.8"
      - 1 : "4.8"
    ▿ color : 3 elements
      - 0 : 1
      - 1 : 2
      - 2 : 3
    ▿ gearbox_id : 1 element
      - 0 : 2
    - is_georgia : false
  </code></pre>