### Description
This PR adds general helper functions to the `x/staking` module that are used in the Evmos `x/vesting` module and originated from Agoric's custom staking module implementation https://github.com/agoric-labs/cosmos-sdk/blob/4085-true-vesting/x/staking/keeper/delegation.go.
- `GetDelegatorUnbonding`
- `GetDelegatorBonded`
- `IterateDelegatorUnbondingDelegations`
- `IterateDelegatorDelegations`
- `IterateDelegatorRedelegations`