Add vintage start and end date to credit transactions

[badgley]

Many issuance and retirement transactions contain information about the start and end date of i) the vintage year and/or ii) the monitoring start / end date. Previously, we've used these data solely to assign credit vintage dates (for example, see https://offsets-db-data.readthedocs.io/en/latest/data-processing.html#an-example).

Some users have requested access these dates. To accommodate those users, we should add vintage_start_date and vintage_end_date to the downloadable version of OffsetsDB.