MBT: Add soft opt-out

[p-offtermatt]

This is an issue that tracks the progress on adding soft opt-out: https://cosmos.github.io/interchain-security/adrs/adr-009-soft-opt-out

Several main tasks should be done:

1. Adjust the Quint model to include the feature
2. Add tests in Quint to test the feature
3. Make sure that the generated traces use the feature in non-trivial ways (or add new generation methods, i.e. invariants, that do)
4. Expand the driver to understand/utilize the new feature

Note: Before starting the work on this task, double-check the status of partial set security https://forum.cosmos.network/t/chips-discussion-phase-partial-set-security/11775 - if work on that is promising, it will likely remove the need to add soft opt-out

[p-offtermatt]

Closing, not necessary since PSS is coming, see #853