[Pools] Refactors and runtime apis for DelegateStake (#4537)

## Runtime Apis
Introduces the following runtime apis to facilitate dapps and wallets in
integrating with the `DelegateStake` functionalities of the pools
(related: https://github.com/paritytech/polkadot-sdk/pull/3905). These
apis are meant to support pool and member migration, as well as lazy
application of pending slashes of pool members.

```rust
fn pool_pending_slash(pool_id: PoolId) -> Balance;
fn member_pending_slash(member: AccountId) -> Balance;
fn pool_needs_delegate_migration(pool_id: PoolId) -> bool;
fn member_needs_delegate_migration(member: AccountId) -> bool;
```

## Refactors
- Introduces newtypes for `Agent`, `Delegator`, `Pool` and
`[Pool]Member`. And refactors `StakeAdapter` and `DelegationInterface`
to accept the above types. This will help make these apis typesafe
against using wrong account type.
- Fixing `DelegationInterface` apis to return optional (instead of
default value if key does not exist).
- Rename struct `Agent` that wraps `AgentLedger` to `AgentOuterLedger`
which is clearer (naming wise) and different from the newtype `Agent`.
- Cleaning up new Pool events (related to `Delegation` feature of pool).

---------

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