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!

首先检查 plugin 当前如何表示和应用单个日期范围保留策略，然后研究 PostgreSQL 17 新增的增量备份支持如何与保留策略交互。完成的标准是支持“7 天、4 周、12 个月、7 年”这样的传统策略，而不要求长期每天执行完整备份。

由索引模型根据 Issue 内容生成。