fix(`/blocks`): cache extrinsic base weight constants to improve performance (#478)

This fixes a regression that happened when v2.1.2 was released. ie: [commit](https://github.com/paritytech/substrate-api-sidecar/commit/5ec24e63d571eaf348eb58053ccc8a7054276bd0). It achieves this by cacheing extrinsic weight constants from the metadata that are used in extrinsic fee calculations.