[lldb-dap] Migrating terminated statistics to the event body.

Per the DAP spec, the event 'body' field should contain any additional data related to the event. I updated the lldb-dap 'statistics' extension into the terminated event's body like:

```
{
  "type": "event",
  "seq": 0,
  "event": "terminated",
  "body": {
    "$__lldb_statistics": {...}
  }
}
```

This allows us to more uniformly handle event messages.

Author: ashgti

lldb/test/API/tools/lldb-dap/terminated-event/TestDAP_terminatedEvent.py

@@ -43,7 +43,7 @@ def test_terminated_event(self):
         self.continue_to_breakpoints(breakpoint_ids)
         self.continue_to_exit()
 
-        statistics = self.dap_server.wait_for_terminated()["statistics"]
+        statistics = self.dap_server.wait_for_terminated()["body"]["$__lldb_statistics"]
         self.assertGreater(statistics["totalDebugInfoByteSize"], 0)
         self.assertGreater(statistics["totalDebugInfoEnabled"], 0)
         self.assertGreater(statistics["totalModuleCountHasDebugInfo"], 0)

lldb/tools/lldb-dap/JSONUtils.cpp

@@ -1526,7 +1526,8 @@ static void addStatistic(lldb::SBTarget &target, llvm::json::Object &event) {
     const char *key = keys.GetStringAtIndex(i);
     FilterAndGetValueForKey(statistics, key, stats_body);
   }
-  event.try_emplace("statistics", std::move(stats_body));
+  llvm::json::Object body{{"$__lldb_statistics", std::move(stats_body)}};
+  event.try_emplace("body", std::move(body));
 }
 
 llvm::json::Object CreateTerminatedEventObject(lldb::SBTarget &target) {