External operator slashes info pallet

[girazoki]

It adds the external operator slashes pallet:

Associated config traits:

• EraIndexProvider: which gives information about active era and when the era started
• Defer Period: a constant that represents the time that we have to cancel a slash
• BondingDuration: a constant that represents the time for which we have to maintain certain structures to be able to prove slashes. Obviously slashes are not maintained longer than the bonding duration.

Extrinsics:

• cancel_deferred_slash: an extrinsic that allows us to cancel an existing slash within the deferred period
• force_inject_slash: an extrinsic that allows us to forcedly inject a slash without having to verify it. Mostly for testing purposes.

Pallet purpose:

• Store a mapping between eraIndex -> vec![Slash], where Slash contains information about the percentage to be slashed, the account, etc.

Testing:
Offences are tested by using both babe and grandpa equivocations, both in rust and typescript

[Agusrodri]

Looks good to me!