The runtime upgrade trait.
Implementing this lets you express what should happen when the runtime upgrades, and changes may need to occur to your module.
fn on_runtime_upgrade() -> Weight
Perform a module upgrade.
This function will be called before we initialized any runtime state, aka
wasn't called yet. So, information like the block number and any other
block local data are not accessible.
Return the non-negotiable weight consumed for runtime upgrade.