(opti) use StorageDoubleMap to store certifications
This could be a performance optimization.
https://paritytech.github.io/polkadot-sdk/master/frame_support/storage/trait.StorageDoubleMap.html
This could be a performance optimization.
https://paritytech.github.io/polkadot-sdk/master/frame_support/storage/trait.StorageDoubleMap.html