Provide a default service for the Hello world example

[valentincanonical]

Provide a default service for the Hello world example. Should probably be done with other examples as well.

• Have you signed the CLA?

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[cjdcordeiro]

this is a bit controversial.

Pebble services do not support one-shot commands like hello, thus why such examples don't have any services. It is completely fine to have a ROCK without any pebble services, if the desired execution mode is leveraging one-shot instructions via pebble exec. having short-lived commands associated with pebble services will cause ROCKs to behave in a weird way since pebble's service restart policies and health checks will be ignored (cause the service won't live for enough time).

Useful reference:

• Pebble does not try to restart service if exited too quickly for the first time pebble#240

PS.: I do think we should support this though, but it is something that needs to be changed on the Pebble side.

[sergiusens]

closing until controversy is resolved