Design Segment document for DynamoDB

Design a standalone document for storing segments in their own documents inside the `environments_v2` table, outside of the `_META` document.

This work should consider:

- indexable segment metadata
- non-indexed fields compression
- migration effort

### Questions

- Should segment overrides still live inside this segment document?
- Current thinking is that for now, yes. Later, no. This should be answered as part of the work in this ticket.