Slashing in the provider chain for consumer chain infractions

[insumity]

Problem

We would like to be able to slash in the provider chain if there's a double-sign infraction in the consumer chain. Currently, this is not possible and that's what led to proposal 818.
However, it is extremely challenging to slash in the producer chain due to a consumer chain infraction because:

• We might have the infraction height in the consumer chain but we need the corresponding producer chain height in order to perform the slashing. The correspondence between consumer- to producer-chain heights stems from the consumer chain. Therefore, if we assume that the consumer-chain binary could be buggy, or even worse malicious, we cannot trust the correspondence between consumer- to producer-chain heights.
• We cannot slash delegators if their voting power did not contribute to the infraction. For example, if a delegator D starts unbonding at height 100 and the infraction occurred at height 101, delegator D should not get slashed.

Closing criteria

The plan is as follows:

1. Investigate if we can have some type of dummy correspondence (e.g., perform slashing in the producer chain when we receive the evidence). Can we do this without unfairly slashing delegators?
2. We have some ideas on how we can utilize light-client headers to verify that a VSCPacket was indeed applied on the consumer chain. We would like to investigate and write an ADR on using those headers in order to be able to increase our confidence in a consumer- to producer-chain height correspondence.