Add support for PostgreSQL 18.

[darkixion]

The function signature for ReplicationSlotAcquire changed and now requires a third parameter. There is also now a macro to get the tuple descriptor attribute.

[ahouska]

Thanks for the pull request. I'll handle the compatibility as soon as I know whether the REPACK CONCURRENTLY command ended up in PG 18 or not. With this command, pg_squeeze would no longer need to deal with the low level stuff. (But it'd still make sense to use pg_squeeze for scheduling.)