Package a summary of the RemoteSettings dumps with the application, so that RemoteSettings clients can look up the last_modified value of a dump without loading the whole JSON dump file. For simplicity, the initial version of `gen_last_modified.py` generates only one entry for the only present use case. A more generic version of the script will be implemented in bug 1719560. Differential Revision: https://phabricator.services.mozilla.com/D119336