Implements a % cap on staking rewards from era inflation (#1660)

This PR implements an (optional) cap of the era inflation that is
allocated to staking rewards. The remaining is minted directly into the
[`RewardRemainder`](https://github.com/paritytech/polkadot-sdk/blob/fb0fd3e6/substrate/frame/staking/src/pallet/mod.rs#L160)
account, which is the treasury pot account in Polkadot and Kusama.

The staking pallet now has a percent storage item, `MaxStakersRewards`,
which defines the max percentage of the era inflation that should be
allocated to staking rewards. The remaining era inflation (i.e.
`remaining = max_era_payout - staking_payout.min(staking_payout *
MaxStakersRewards))` is minted directly into the treasury.

The `MaxStakersRewards` can be set by a privileged origin through the
`set_staking_configs` extrinsic.

**To finish**
- [x] run benchmarks for westend-runtime

Replaces https://github.com/paritytech/polkadot-sdk/pull/1483
Closes https://github.com/paritytech/polkadot-sdk/issues/403

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Co-authored-by: command-bot <>