Add export_started_at to Stats response

After a graph is exported, Software Heritage archive will continue to grow and change. Currently clients have no easy way to know when was made the export available from a swh.graph server. Thus making it harder to figure out was has been added to swh.storage since.

Therefore, we add a new field to the response of the Stats API, “export_started_at”. The field is in “seconds since epoch” like other time values currently in the protocol.

The value is taken from the meta/export.json file. The meta directory needs to be a sibbling to the compressed directory hosting webgraph data file.

Closes #4779 (closed)