Add Probes.stop behavior

[tombruijn]

As agreed upon in https://github.com/appsignal/integration-guide/pull/124, add a function to manually stop the minutely probes.

This may help with issue appsignal/appsignal-nodejs#418

To do

• Implement a public Probes.stop function.
• Call Minutely.stop in Appsignal.stop