feat: added ability to configure SDK client to use an external context

[carlosroman]

At the moment there is no way to stop the harvester from running as it uses its own background context. Added a new config variable so a context can be passed to the client. The "tick" in harvester.harvestRoutine can now be cancelled by the user.

This resolves #26.

[CLAassistant]

Thank you for your submission! We really appreciate it. Like many open source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution.
_{You have signed the CLA already but the status is still pending? Let us recheck it.}