Make callback URL configurable
Problem: A CiviCRM instance is running on a private network with a Proxy that is set up (using CiviProxy) to handle necessary communication with the outside world. Callbacks that are automatically configured by the extension will not reach the CiviCRM instance.
Proposed solution: Add an option to the sparkpost admin settings form that allows the callback URL to be specified so that when the form is submitted and the setting is create/refreshed it is set to an appropriate URL on the proxy.
Happy to submit a PR along these lines if this sounds like an acceptable approach to you.