Simple "Cron" style workflows

[JoeHowarth]

Allowing workflows to be kicked off by events other than VAAs coming in through the spy will allow more interesting utility plugins to be written. Examples include:

• audit plugin: periodically check that transactions haven't been reverted and re-kickoff original workflow if they are
• wallet rebalancer plugin: periodically check wallet balances and send wh transfers to balance them
• arbitrary cranks: many on-chain programs need to be cranked periodically. This allows integrators to incorporate these services into the relayer engine to take advantage of its wallet management etc.

This could be as simple as a mapping like:

interface Job {
    interval: number, // ms 
    fn: () => Workflow,
}

interface Plugin {
    jobs(): Job[]
    ...
}

[JoeHowarth]

Partially fixed by #17

[gabzim]

I believe this is out of scope @JoeHowarth , would it be ok to close this?