Tvl pool staking (#1322)

What does this PR do?
- Introduced the TotalValueLocked storage for nomination-pools. 
- introduced a slashing api in mock.rs 
- additional test for tracking a slashing event towards a pool without
sub-pools
- migration for the nomination-pools (V6 to V7) with
`VersionedMigration`

Why are these changes needed?
this is the continuation of the work by @kianenigma in this
[PR](https://github.com/paritytech/substrate/pull/13319)

How were these changes implemented and what do they affect?
- It's an extra StorageValue that's modified whenever funds flow in or
out of staking for any of the `bonded_account` of `BondedPools`
- The `PoolSlashed`event is now emitted even when no `SubPools` are
found

Closes https://github.com/paritytech/polkadot-sdk/issues/155
KSM: HHEEgVzcqL3kCXgsxSfJMbsTy8dxoTctuXtpY94n4s8F4pS

---------

Co-authored-by: Liam Aharon <liam.aharon@hotmail.com>
Co-authored-by: Kian Paimani <5588131+kianenigma@users.noreply.github.com>
Co-authored-by: Ankan <10196091+Ank4n@users.noreply.github.com>
Co-authored-by: Ankan <ankan.anurag@gmail.com>
Co-authored-by: command-bot <>

substrate/primitives/staking/src/lib.rs

@@ -121,10 +121,12 @@ pub trait OnStakingUpdate<AccountId, Balance> {
 	/// * `slashed_active` - The new bonded balance of the staker after the slash was applied.
 	/// * `slashed_unlocking` - A map of slashed eras, and the balance of that unlocking chunk after
 	///   the slash is applied. Any era not present in the map is not affected at all.
+	/// * `slashed_total` - The aggregated balance that was lost due to the slash.
 	fn on_slash(
 		_stash: &AccountId,
 		_slashed_active: Balance,
 		_slashed_unlocking: &BTreeMap<EraIndex, Balance>,
+		_slashed_total: Balance,
 	) {
 	}
 }