Introduce Payees and PayoutDestination with Split variant

[rossbulat]

Tracking issue: paritytech/polkadot-sdk#410
Polkadot companion: paritytech/polkadot#7425

This PR is part 1 of a lazy migration to update reward destinations, which removes Controller, refactors RewardDestination into a new PayoutDestination enum, and adds an additional Split enum for the ability to compound a part of rewards and have the rest as free balance.

• Create PayoutDestination with new variants and introduce Payees storage item to replace Payee.
• set_payee removes Payee (RewardDestination) and inserts to Payees (PayoutDestination)
• Introduce update_payee call to lazily migrate from Payee to Payees storage item.
• Implement Split logic on payout_stakers with getter that migrates Payee to new Payees storage.
• Review staking benchmarks and ensure they are worst case scenario (Split, existing Payee).
• Add tests again for fee-less update_payee.
• Add tests for get_payout_destination, ensure Payees is populated.

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