[SE-4235] Add missing parameter processors code
Somewhere between versions 2.0.0 and 2.1.0, the code to handle external parameter processors was dropped. This PR adds it back. Without this, external processors that are specified in the [parameter processors settings](https://github.com/edx/xblock-lti-consumer#configuring-the-parameter-processors-settings) is not called.
*Jira Tickets:* (https://openedx.atlassian.net/browse/OSPR-5686)
*Sandbox URL:* TBD
1. Install this branch on your xblock runtime environment (for example xblock-sdk).
2. Test the parameter processors from tahoe-lti as per the [test instructions provided](https://github.com/appsembler/tahoe-lti#development-guide)
3. Verify that the external parameter processors are being called correctly.
[ ] @gabor-boros
[ ] edX reviewer TBD
I’m assigning this to you since you asked Giovanni to take a look.