Fuzzing: add an option to submit a random transaction sampled from all possible ones for all agents

[davidsiska-vega]

Our fuzzing agents can be a bit boring (as evidenced by a crash that needed a real human on fairground).

I am wondering if we could add a feature where every fixed number of steps any of our agents will sample from all possible transactions (and I really mean all possible ones, validator joining, market creation, order submissions, cancellations etc.) and then submit random values, ideally within valid range except perhaps for things like market ids which need to make sense for this to have value.

Ideally, there would be a way of automatically generating all the possible transactions and ranges from protos - then we just need to enumerate them. We don't want to be maintaining this by hand.

So far for discussion only.