Unify staking related runtime APIs

[gpestana]

Currently, there are a few staking related runtime APIs exposed by the clients (e.g. paritytech/substrate#13119). However, the current runtime APIs are i) not exhaustive and ii) not general/unified.

We should design and implement a set of runtime APIs to query staking related data that covers the needs of wallets and other UIs and that are generic enough to decouple most of the future underlying logic/storage changes from UIs to improve interface stability in the future.

[kianenigma]

A great practice here would be to make sure whatever we implement is compatible with https://docs.moonbeam.network/builders/pallets-precompiles/pallets/staking/

[gpestana]

We should consider designing and implementing View Functions for staking when they are available, perhaps it is even more relevant and useful than unifying the staking runtime APIs