Allow to specify _part_offset in normal projection

Why it matters

This feature enables the foundation for building projection indexes, enhancing query performance by allowing more efficient part-level data identification and processing. It complements related improvements and helps address performance issues noted in existing tickets.

How to use it

Users can specify the _part_offset within their normal projection definitions to leverage this feature. It works in conjunction with other changes like those discussed in issue #58224 to optimize projection indexing.