Allow privileged virtual bond in Staking pallet (#3889)

This is the first PR in preparation for
https://github.com/paritytech/polkadot-sdk/issues/454.

## Follow ups:
- https://github.com/paritytech/polkadot-sdk/pull/3904.
- https://github.com/paritytech/polkadot-sdk/pull/3905.

Overall changes are documented here (lot more visual ):
https://hackmd.io/@ak0n/454-np-governance

[Maybe followup](https://github.com/paritytech/polkadot-sdk/issues/4217)
with migration of storage item `VirtualStakers` as a bool or enum in
`Ledger`.

## Context
We want to achieve a way for a user (`Delegator`) to delegate their
funds to another account (`Agent`). Delegate implies the funds are
locked in delegator account itself. Agent can act on behalf of delegator
to stake directly on Staking pallet.

The delegation feature is added to Staking via another pallet
`delegated-staking` worked on
[here](https://github.com/paritytech/polkadot-sdk/pull/3904).

## Introduces:
### StakingUnchecked Trait
As the name implies, this trai...