Make `applyCostModelParams` code more explicit

We have to allow dynamic updates of the cost model parameters, and [currently this is done](https://github.com/IntersectMBO/plutus/blob/d319fe5844cf9fc791510d7b63bf76322a97d2d5/plutus-core/plutus-core/src/PlutusCore/Evaluation/Machine/CostModelInterface.hs#L37) by converting our internal cost model structures to JSON, flattening that into a hashmap using functions from Aeson, merging that with the new data, and then reversing the process. This is very complicated and we should do something better. At the very least we should be able to dig out what Aeson is actually doing and do it directly on the level of our datatypes. See also #5945 (which we should do first).