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/frame/staking/src/mock.rs

@@ -278,6 +278,7 @@ impl OnStakingUpdate<AccountId, Balance> for EventListenerMock {
 		_pool_account: &AccountId,
 		slashed_bonded: Balance,
 		slashed_chunks: &BTreeMap<EraIndex, Balance>,
+		_total_slashed: Balance,
 	) {
 		LedgerSlashPerEra::set((slashed_bonded, slashed_chunks.clone()));
 	}