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

src/tool_writeout.h

@@ -32,6 +32,8 @@ typedef enum {
   VAR_CERT,
   VAR_CONNECT_TIME,
   VAR_CONTENT_TYPE,
+  VAR_CONN_ID,
+  VAR_EASY_ID,
   VAR_EFFECTIVE_FILENAME,
   VAR_EFFECTIVE_METHOD,
   VAR_EFFECTIVE_URL,