Create operation and transaction for custom smart contract calls

[nguyer]

Invoking a smart contract should be tracked by an Operation and a Transaction in FireFly.

[awrichar]

This item turns out to be difficult because we have no guarantees about what events may be emitted as a result of a custom smart contract call. This means many of the existing fields of Transaction can't be filled in, which means re-thinking the purpose of Transaction and how it fits into the landscape in light of all the custom onchain work. #408 addresses the groundwork needed before this issue can be solved.