Partial Set Security: Add MsgOptIn & MsgOptOut

[insumity]

Problem

Add the MsgOptIn and MsgOptOut messages that would be used for Partial Set Security (see ADR).

Closing criteria

1. Investigate on whether we can read the votes of proposals to automatically opt in and if so implement this aspect as well (see Problem Details).
2. State should be updated to keep track of all the validators that opted in (and when they opted in).
3. State should be updated to remove validators when they opt out.
4. Provide keeper methods as outlined in the ADR.
5. Thoroughly test.

Problem details

It's not clear whether we can use the votes of a ConsumerAdditionProposal to automatically opt in a validator. This is because the Tally method deletes the votes after reading them. This also makes sense; There's no reason to keep the votes around after tallying them. Therefore, we wouldn't be able to extract any information on who voted Yes after a proposal has passed through GetVotes.