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 <[email protected]>
Co-authored-by: Kian Paimani <[email protected]>
Co-authored-by: Ankan <[email protected]>
Co-authored-by: Ankan <[email protected]>
Co-authored-by: command-bot <>