[WIP] RFC-0097 Implementation to Decrease Unbonding Time

[rossbulat]

This PR aims to implement RFC-0097, that introduces a flexible unbonding mechanism to decrease the time stakers have to wait to unbond by as much as 26 eras, resulting in 2 eras / 2 days on Polkadot in the best case scenario.

Commit Log

• Introduce MinSlashableShare, UnbondPeriodUpperBound and UnbondPeriodLowerBound parameters as genesis storage items.
• Define new migration to set initial parameter values in storage.
• Record how much stake was used to back the lowest-backed 1/3 of validators, for the last UnbondingPeriodUpperBound eras. This LowestThirdTotalStake storage record can be updated when a new planned era is triggered, more specifically in the store_stakers_info function.
• Implement a get_min_lowest_third_stake function that gets the lowest third stake entry over UnbondPeriodUpperBound eras.
• Create pallet function get_quick_unbond_max_unstake(), that will essentially return MinSlashableShare * min_lowest_third_stake, where min_lowest_third_stake is determined by the above storage addition. Utilising this new storage item is of course less weight heavy than iterating through ErasStakersPaged.
• Calculate unbonding_time_delta - this can use the following formula to determine a concrete unbond block number:
• Calculate back_of_unbonding_queue_block_number.
• Calculate unbonding_block_number and reflect in unlock chunks.

More to come...

Post Implementation Checklist

• Add migration to runtimes.

[paritytech-cicd-pr]

The CI pipeline was cancelled due to failure one of the required jobs.
Job name: test-linux-stable 2/3
Logs: https://gitlab.parity.io/parity/mirrors/polkadot-sdk/-/jobs/7347472