[FEATURE] Include OSM replication timestamp in mbtiles metadata

[msbarry]

It would be helpful if the tilejson output (which typically copies all key/value pairs from the mbtiles metadata table) included the timestamp/version of input source data to help you understand what you're seeing in the output. To do this let's add planetiler:osm:timestamp metadata attribute that copies the OSM replication timestamp to output file.

Alternatives Considered

Include the input data source version/timestamp in description or version field. This stays within the tilejson/mbtiles spec, but is harder for clients to parse. Mbtiles servers seem to just pass any unknown fields through from metadata to tilejson, so custom keys seem like a better option.