[SPEC] Challenge-response misbehaviour report

[1xstj]

Misbehaviour reports

We need to support misbehaviour reports that follow a challenge-reponse model.

Steps:

1. Submit a misbehaviour report (report Alice as the offending validator)
2. Alice has T::ResponseBlocks blocks to prove that offence report is false
3. After T::ResponseBlocks, either Alice or offence reporter is slashed.