refactor: move all cache parameter defaults to the library

[wbruna]

Simplifies cache initialization for library users.

[wbruna]

@rmatif and @leejet , the --cache-preset handling from 186038e apparently got discarded before #1066 got merged as 298b110 . The option is validated and written to the cache_preset field, but the value isn't used anywhere.

Fixing it isn't straightforward because the needed functions (get_scm_preset, get_preset_threshold, etc) on cache-dit.hpp are not part of the library API. So my idea is, on top of this PR here:

• add a new field for the preset to the sd_cache_params_t struct
• initialize the four relevant fields to specific invalid values to distinguish them from values explicitly set through --cache-options (I believe negatives would work);
• during inference, replace the negatives (and an empty scm array) with the appropriate values according to the preset.

But... since this bug went off the radar for over two months, I'm wondering if it's really worth the effort, or if we should just remove the flag instead. What do you think?