Add Update and Delete capabilities for LineItems

[Greyvend]

Hello Dmitry!

First of all, thanks a lot for your implementation of the LTI protocol in Python.

As we've started to adopt the LTI integration in our Tool, we've noticed some of the important LTI documented capabilities missing in the library. One of those is the support for modification and deletion of existing line items. This PR aims to add just that.

What was done in this PR:

1. Changed the interface of the ServiceConnector.make_service_request method to support other HTTP methods (here).
2. Added two new methods to the AssignmentsGradesService: update_lineitem, delete_lineitem (here).
3. Added tests for these methods (here). I've also tested them with a local Canvas instance.

These actions are documented in the LTI page.