Set merchant callbacks

[DanielePalombo]

Allow the extension to set the oauth callbacks.

Using the setCallbacks and getCallbacks APIs the extension should be able to detect and set them.

• Create a service that through the getCallbacks will return all the already set callbacks, providing URL and type.
• Create a service that Given URLs and type callbacks will update the value of the callback using the setCallbacks API.
• Create a CRUD that using the above services will let the user see and update the callbacks.
• Update the README with instructions on how to configure the callbacks for solidus.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It might be closed if no further activity occurs. Thank you for your contributions.