Draft self-stake missing rewards calculation

[paulormart]

This example calculates the self-stake missed rewards on asset-hub-kusama between eras 8662 and 8757. It will also output a csv file containing all validators, rewards and respective reward destination account.

Run with cargo run --example=storage_calculate_self_stake_rewards

Running `target/debug/examples/storage_calculate_self_stake_rewards`
Iterating over 307230 blocks
Processed 0%, 307230 blocks still to go.
Processed 10%, 276507 blocks still to go.
Processed 20%, 245784 blocks still to go.
Processed 30%, 215061 blocks still to go.
Processed 40%, 184338 blocks still to go.
Processed 50%, 153615 blocks still to go.
Processed 60%, 122892 blocks still to go.
Processed 70%, 92169 blocks still to go.
Processed 80%, 61446 blocks still to go.
Processed 90%, 30723 blocks still to go.
Calculated total self-stake rewards: 336.645890068692 KSM for 876 validators (488.405832073s)

Output file
self_stake_rewards.csv