Add CLI command for estimating gas cost of a transaction

[tjanez]

SUMMARY

When a user creates a transaction using CLI, he needs to specify transaction.fee.gas, i.e. the maximum amount of gas a transaction can spend, but he has to estimate the gas cost of a transaction manually.

We already support automatically estimating a transaction's gas cost:
https://github.com/oasislabs/oasis-core/blob/ffa12899c2fc44aed9848ab87d6709a8805b2bc3/go/consensus/api/api.go#L89-L90

We just need to expose it via CLI.

ISSUE TYPE

• Feature Idea