Go middleware API

[Jeroen Rinzema]

A Go middleware API should be introduced to allow additional features to be developed upon Maestro. Implementations such as the open tracing API are great use-cases.

// example middleware initialization
maestro.New(
  maestro.WithMiddleware(tracing.New())
)

Middlewares should be able to register to various events inside maestro such as BeforeFlow, AfterFlow and OnFlowSuccess.