Tappx Adapter: Add GPID support

Author: AntoxaAntoxic

src/main/java/org/prebid/server/bidder/tappx/TappxBidder.java

@@ -36,7 +36,7 @@
 
 public class TappxBidder implements Bidder<BidRequest> {
 
-    private static final String VERSION = "1.4";
+    private static final String VERSION = "1.6";
     private static final String TYPE_CNN = "prebid";
 
     private static final TypeReference<ExtPrebid<?, ExtImpTappx>> TAPX_EXT_TYPE_REFERENCE =

src/test/resources/org/prebid/server/it/openrtb2/tappx/test-auction-tappx-request.json

@@ -12,7 +12,8 @@
           "tappxkey": "pub-12345-android-9876",
           "endpoint": "test",
           "bidfloor": 1.5
-        }
+        },
+        "gpid": "/19968336/header-bid-tag-0"
       }
     }
   ],

src/test/resources/org/prebid/server/it/openrtb2/tappx/test-tappx-bid-request.json

@@ -15,7 +15,8 @@
           "tappxkey": "pub-12345-android-9876",
           "endpoint": "test",
           "bidfloor": 1.5
-        }
+        },
+        "gpid": "/19968336/header-bid-tag-0"
       }
     }
   ],