[PATCH] Add `truncate` option to subscription commands

-hackers,

Enclosed find a patch to add a “truncate” option to subscription commands.

When adding new tables to a subscription (either via `CREATE SUBSCRIPTION` or `REFRESH PUBLICATION`), tables on the target which are being newly subscribed will be truncated before the data copy step. This saves explicit coordination of a manual `TRUNCATE` on the target tables and allows the results of the initial data sync to be the same as on the publisher at the time of sync.

To preserve compatibility with existing behavior, the default value for this parameter is `false`.

Best,

David