contact: use last_seen column

It was there since the C core, labeled with "/* last_seen is for
future use */" but never actually used. The comment was lost during
the translation from C to Rust.

deltachat-ffi/deltachat.h

@@ -4382,6 +4382,16 @@ uint32_t        dc_contact_get_color         (const dc_contact_t* contact);
  */
 char*           dc_contact_get_status        (const dc_contact_t* contact);
 
+/**
+ * Get the contact's last seen timestamp.
+ *
+ * @memberof dc_contact_t
+ * @param contact The contact object.
+ * @return Last seen timestamp.
+ *     0 on error or if the contact was never seen.
+ */
+int64_t         dc_contact_get_last_seen     (const dc_contact_t* contact);
+
 /**
  * Check if a contact is blocked.
  *