dApp staking v3 - storing reward per staked ASTR instead of the reward pool

[Dinonard]

Description

To simplify off-chain calculations, we could store reward per staked ASTR value into the reward info, instead of the overall reward pool.

This would help since off-chain integrations would not need to worry about matching Rust/polkadot-sdk's rounding logic.

It's a minor improvement suggestion.