Add configuration for extra middleware classes

Contains configuration changes necessary for changing setting added in edx/edx-platform#16108.

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Make sure that the following steps are done before merging:

• [ ] A DevOps team member has approved the PR.

• [ ] Are you adding any new default values that need to be overridden when this change goes live? If so:

• [x] Update the appropriate internal repo (be sure to update for all our environments)

• [ ] If you are updating a secure value rather than an internal one, file a DEVOPS ticket with details.

• [ ] Add an entry to the CHANGELOG.

• [ ] If you are making a complicated change, have you performed the proper testing specified on the [Ops Ansible Testing Checklist](https://openedx.atlassian.net/wiki/display/EdxOps/Ops+Ansible+Testing+Checklist)? Adding a new variable does not require the full list (although testing on a sandbox is a great idea to ensure it links with your downstream code changes).