Custom ISI/expressions in executor

[mversic]

I'd like if we could make one integration test that covers this use case.

Rationale

Previously we had expression system but we removed it for the sake of simplicity and promised that it would become possible for blockchain admins to roll out their own expression system by upgrading executor

[mversic]

Since ISIs get evaluated only in Executor, it's possible for admin to roll out their own custom expanded instruction set. All custom instructions must evaluate into ISI since that is what Iroha can operate on (it's our RISC) but client can have much more rich API to work with

[AlexStroke]

covered in tests/integration