feat(sdk): improve free-form JSON ergonomics

[matoous]

Update the code generator and SDK runtime so free-form object schemas are exposed as a JsonObject helper instead of raw JsonDocument/JsonElement shapes, improving the authoring experience for payloads like Apple Pay / Google Pay data, metadata, tags, and other additionalProperties: true maps while keeping the wire format unchanged. This also updates generated dictionary-only models to inherit from JsonObject and adds regular generator and SDK tests covering the new mapping, serialization, and parsing behavior.

  var readerCheckout = new CreateReaderCheckoutRequest
  {
      Description = "Coffee",
      TotalAmount = new CreateReaderCheckoutRequestTotalAmount
      {
          Currency = "EUR",
          MinorUnit = 2,
          Value = 450,
      },
      Affiliate = new CreateReaderCheckoutRequestAffiliate
      {
          AppId = "com.example.pos",
          ForeignTransactionId = "order-123",
          Key = "affiliate-key",
          Tags = new JsonObject
          {
              ["store_id"] = "berlin-01",
              ["cashier_id"] = 42,
              ["sandbox"] = true,
          },
      },
  };