[spark] Adjust ExpireSnapshotsProcedure param type

[Askwang]

Purpose

• change param older_than type from TimeStampType to StringType, TimeStampType is not easy to understand and use.
• add new param time_retained, which can call expire_snapshots periodically. older_than is suitable for one-time call.

Tests

API and Format

Documentation

[Askwang]

help cc @JingsongLi