Refactor SLH-DSA C integration

[xuganyu96]

SLH-DSA (specifically the slhdsa-c impl) was integrated into liboqs with the 0.15.0 release. However, a lot of integration logic is inconsistent with all other algorithm/implementation integrations, and integration metadata is not stored in the same copy_from_upstream.yml datasheet as all other implementations.

This issue tracks the ongoing effort to refactor the existing SLH-DSA C integration scaffolding. I am aiming to avoid any changes that affects external users.