Introduces: Delegated Staking Pallet (#3904)

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

## Also see
- **Precursor** https://github.com/paritytech/polkadot-sdk/pull/3889.
- **Follow up** https://github.com/paritytech/polkadot-sdk/pull/3905.

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



## Changes
### Delegation Interface
Provides delegation primitives for staking. 

Introduces two new roles:
- Agent: These are accounts who receive delegation from other accounts
(delegators) and stakes on behalf of them. The funds are held in
delegator accounts.
- Delegator: Accounts who delegate their funds to an agent authorising
them to use it for staking.

Supports
- A way for delegators to add or withdraw delegation to an agent.
- A way for an agent to slash a delegator during a slashing event.

### Pallet Delegated Staking
- Implements `DelegationInterface`.
- Lazy slashing: Any slashes to an Agent is posted in a ledger but not
immediately slashed. The agent can call
`DelegationInterface::delegator_slash` to slash the member and clear the
corresponding slash from its ledger.
- Consumes `StakingInterface` to provide `CoreStaking` features. In
reality, this will be `pallet-staking`.
- Ensures bookkeeping for agent and delegator are correct but leaves the
management of reward and slash logic upto the consumer of this pallet.
- While it does not expose any calls yet, it is written with the intent
of exposing these primitives via extrinsics.

## TODO
- [x] Improve unit tests in the pallet.
- [x] Separate slash reward perbill for rewarding the slash reporters?
- [x] Review if we should add more events.

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Co-authored-by: Kian Paimani <[email protected]>
Co-authored-by: Gonçalo Pestana <[email protected]>
Co-authored-by: georgepisaltu <[email protected]>