Bug - incorrect invoke transaction trace structure

[dkatzan]

The go struct defined for the trace of an invoke transaction is incorrect, causing to missing data while parsing API results

specifically, the following field ExecInvocation struct defined here and used here
contains 2 inner fields, while the node result is slightly different, resulting with the inner FnInvocation to always be empty

Also, the relevant TestSimulateTransaction in mainnet is broken due to multiple reasons, part of them discussed here

I already have a draft work in progress, and will be happy to open a PR to fix these issues soon