Staking Reward Pool implementation

[shannonwells]

Story

In order to pay out Staking Rewards, there needs to be a Reward Pool calculated per Staking Era to pay rewards from.

Acceptance Criteria

• RewardPoolInfo is in Capacity's pallet storage
• Reward Pool size is obtained from the StakingRewardProvider that is configured by Capacity pallet runtime/mocks
• Every Era, a new RewardPoolInfo entry is appended to storage with the total reward pool amount and zero for unclaimed_balance.

Blocked by

• Staking Rewards Eras basics #1567
• Implement StakingRewardsProvider traits in runtime and mocks #1572
• Probably something else I haven't thought of

Not included

• Rewards payouts
• Rewards history rollover i.e. garbage collection
• Updating StakingRewardInfo whenever someone stakes or unstakes

[shannonwells]

Finished in PR #1710