[Feature] Enable pipeline validation upon startup

[Daafmans]

As someone with little experience with Brighter, setting up a pipeline correctly can be a little tricky. For example, it's easy to mistakenly mix sync and async handlers, subscriptions, policies, etc. When building a consumer, Brighter informs me of these issues when I start everything up and send a message to my broker for my application to consume.

Is it possible to implement something that forces Brighter to validate pipelines at startup? Perhaps something configurable so as to only run in certain environments (like on a local machine), to not impact startup times in production?

This would cut down the time for each cycle in the "build everything, run everything, see problem, fix problem, try again"-cycle

[iancooper]

Thanks @Daafmans. I think it's a useful suggestion as the more help we can give with configuration the better. @preardon @holytshirt thoughts?