perf: add pre_cache_exams_for_course API call

[ormsbee]

Description:

This is to address the performance issue when outline page loading in Studio causes multiple calls to get_exam_by_content_id() for every subsection in the course. On one test course, this resulted in about a hundred separate queries, despite the fact that only two of the subsections had corresponding ProctoredExam models.

This is a WIP, as I still need to add tests.

Pre-Merge Checklist:

• Updated the version number in edx_proctoring/__init__.py and package.json if these changes are to be released.
• Described your changes in CHANGELOG.rst
• Confirmed Github reports all automated tests/checks are passing.
• Approved by at least one additional reviewer.

Post-Merge:

• Create a tag matching the new version number.