lib: add CURLINFO_CONN_ID and CURLINFO_XFER_ID

- add an `id` long to Curl_easy, -1 on init
- once added to a multi (or its own multi), it gets
  a non-negative number assigned by the connection cache
- `id` is unique among all transfers using the same
  cache until reaching LONG_MAX where it will wrap
  around. So, not unique eternally.
- CURLINFO_CONN_ID returns the connection id attached to
  data or, if none present, data->state.lastconnect_id
- variables and type declared in tool for write out

Closes #11185

lib/getinfo.c

@@ -415,6 +415,13 @@ static CURLcode getinfo_offt(struct Curl_easy *data, CURLINFO info,
   case CURLINFO_RETRY_AFTER:
     *param_offt = data->info.retry_after;
     break;
+  case CURLINFO_XFER_ID:
+    *param_offt = data->id;
+    break;
+  case CURLINFO_CONN_ID:
+    *param_offt = data->conn?
+                  data->conn->connection_id : data->state.recent_conn_id;
+    break;
   default:
     return CURLE_UNKNOWN_OPTION;
   }