Ergonomic MST

[mversic]

Ideas on how can we make MST more ergonomic:

1. All smart contracts for MST should be registered from Executor, i.e. be put in default executor migration
2. I think we need an API to fetch all currently voted for transactions, i.e. pending transactions
3. we need a way to expire MST that have not been active for a while
4. We can define custom instructions for dealing with MST and also have them defined as part of default executor

Additionally, we should consider adding (behind a feature flag "mst") a specialized API to both iroha client and cli for dealing with MSTs.

[mversic]

actually, instead of doing it during executor migration as was suggested in 1. they should be registered in the genesis

[s8sato]

Remaining issues after #5027:

• custom instructions to encapsulate multisig operations
• migrate shell script tests to pytests
• regulate multisig account registrations?
• make the default multisig ttl indefinite?