Support traditional retention policies

Currently, I can only find a way to retain a single date range of backups. (pointers to the contrary welcome!)

In a production deployment, our business quite often requires something like 1 year or longer retention, but keeping 1 (or 7) years of daily FULL backups would be quite cost prohibitive!

Please consider adding support for a more traditional retention policy like:

"7 days, 4 weeks, 12 months, 7 years"

Additionally, this plays well with the new incremental backup features available in Pg17!

Start by reviewing how the plugin currently represents and applies a single date-range retention policy, then examine how the new PostgreSQL 17 incremental backup support interacts with retention. Done means supporting a traditional policy such as “7 days, 4 weeks, 12 months, 7 years” without requiring long-term daily full backups.

Written by the indexing model from the issue text.