Add support for background execution on application services

[garciparedes]

Proposed solution:

The proposed solution is to allow to pass the @reboot label to the @enroute.periodic.event decorator, which means that the handling function will be executed only once, at microservice's starting time.

Note that this handling function could be running indefinitely thanks to infinite loops or another related strategies.

class MyService(Service):

    @enroute.periodic.event("@reboot")
    async def task(self, request: Request) -> None:
        print("Running task only once...")