Extrinsic to restore corrupt staking ledgers (#3706)

This PR adds a new extrinsic `Call::restore_ledger ` gated by
`StakingAdmin` origin that restores a corrupted staking ledger. This
extrinsic will be used to recover ledgers that were affected by the
issue discussed in
https://github.com/paritytech/polkadot-sdk/issues/3245.

The extrinsic will re-write the storage items associated with a stash
account provided as input parameter. The data used to reset the ledger
can be either i) fetched on-chain or ii) partially/totally set by the
input parameters of the call.

In order to use on-chain data to restore the staking locks, we need a
way to read the current lock in the balances pallet. This PR adds a
`InspectLockableCurrency` trait and implements it in the pallet
balances. An alternative would be to tightly couple staking with the
pallet balances but that's inelegant (an example of how it would look
like in [this
branch](https://github.com/paritytech/polkadot-sdk/tree/gpestana/ledger-badstate-clean_tightly)...