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 <>

polkadot/runtime/westend/src/lib.rs

@@ -1508,6 +1508,7 @@ pub mod migrations {
 		paras_registrar::migration::VersionCheckedMigrateToV1<Runtime, ()>,
 		pallet_nomination_pools::migration::versioned_migrations::V5toV6<Runtime>,
 		pallet_referenda::migration::v1::MigrateV0ToV1<Runtime, ()>,
+		pallet_nomination_pools::migration::versioned_migrations::V6ToV7<Runtime>,
 	);
 }