fix: Unicode course data causes errors when sending to ClickHouse

[bmtcril]

Description: Fixes unicode errors when sending to ClickHouse by explicitly encoding the CSV output as utf-8.

ISSUE:
Closes: #22

Installation instructions: In a Tutor Aspects local setup you can:

• Change OPENEDX_EXTRA_PIP_REQUIREMENTS for this package to "git+https://github.com/openedx/openedx-event-sink-clickhouse.git@bmtcril/fix_unicode",

Testing instructions:

After installing from this branch:

• Rebuild the openedx image: tutor images build openedx
• tutor local start -d
• Create some unicode in your course name, problem / video block names, etc.
• Publish the course
• Browse the course, watch the video or try the problem
• Confirm in the cms-worker logs that there are no errors
• Confirm in ClickHouse that the changes appear correctly in the event_sink.course_overviews / event_sink.course_blocks tables (note that a lot of database tools don't handle utf-8 well, if there are "weird characters" in the output you can assume this passes and go to the next step)
• Check Superset to make sure that the UTF-8 characters show up correctly in the reports and filters