fix: Update fee calc to use `system::constants::BlockWeights.per_class.normal.base_extrinsic` (#388)

* fix: Update fee calc to use system::blockWeight

* WIP

* Use blockweights constant in metadata

* Add comment

* Bump deps

* New metadata WIP

* Add tests to show how constants are used

* Pre-merge

* Ensure correct constants based on runtime version

* Grammar

* Update specs