Is your feature request related to a problem? Please describe.
The current types of cost progression is somewhat limiting, and introducing new types in code whenever new use cases pop up is undesirable. Even with new types akin to those that already exist, it would not be possible to simply make small adjustments.

Describe the solution you'd like
Define the cost progression for unlocking slots as a list of numbers describing the cost for each sequential slot unlock. This would ideally be done as a config option if well supported. Replaces config options costProgression and costMultiplier.

Describe alternatives you've considered
Instead of a config option, it could be defined as a standalone data pack json file. However the cost progression list does not fit well with the datapack system of overriding or accumulating data across overlapping datapacks.

Additional context
This change would not be backwards-compatible with previous configurations, making this feature relevant when other backwards-incompatible changes are present.
A few templates for cost progression would be appropriate to put somewhere where this feature is explained, such as on the curseforge page.