[contracts] Adapt storage reading host functions to Weights V2 (#12976)

* update RuntimeCosts to Weights V2, update tests

* improve docs

* clearer naming and docs to compat_weight helper

* Apply suggestions from code review

Co-authored-by: Alexander Theißen <[email protected]>

* save before master merge

* HostFnWeights to Weight

* added to_weight! macro

* Apply suggestions from code review

Co-authored-by: Alexander Theißen <[email protected]>

* RuntimeCosts::ChainExtension to weight_v2

* chain extension to weight v2

Co-authored-by: Alexander Theißen <[email protected]>