[Tagging] Enforce typing and encapsulation for tagging APIs

[pomegranited]

Story

"As an Open edX developer, I want to take advantage of python static type checking wherever possible, to reduce runtime errors in my code."
"As an Open edX developer, I want to isolate the use of the tagging libraries to their python APIs to improve encapsulation and reduce tight coupling to the underlying implementation."

Description

cf openedx/openedx-platform#32661 (comment)

Completion criteria

• Add openedx.features.content_tagging to mypy.ini so you get static type checking.
• (stretch goal) Add mypy.ini support to https://github.com/openedx/openedx-learning so you get static type checking on openedx_tagging (minimum), and openedx_learning (if possible).
• Add openedx.features.content_tagging to setup.cfg isolated_apps so you can make sure all other parts of the code are only importing from content_tagging.api.
• Add openedx_tagging.core.tagging to setup.cfg isolated_apps so you can make sure we're only importing from the oel_tagging.api.

Documentation updates & improvements criteria

• N/A

Relevant PRs/repositories

• https://github.com/openedx/edx-platform
• https://github.com/openedx/openedx-learning

[bradenmacdonald]

Partially done via openedx/openedx-core#73
But we still need to turn on mypy checking of the corresponding code in edx-platform