feat: adds get_containers and get_container_by_key to the authoring API [FC-0083]

[pomegranited]

Adds:

• get_containers -- needed by content search API.
• get_container_by_key -- reduces queries performed by content libraries API

And increases test coverage for the units API.

Builds on #278
Testing instructions: see openedx/edx-platform#36371

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[rpenido]

LGTM 👍
Thank you for your work, @pomegranited!

• I tested this using the instructions from openedx/edx-platform#36371
• I read through the code
• I checked for accessibility issues
• Includes documentation

[navinkarkera]

@pomegranited 👍 Looks good.

• I tested this: (describe what you tested)
• I read through the code
• I checked for accessibility issues
• Includes documentation

[pomegranited]

@ormsbee @kdmccormick FYI for these added API methods, and a possible addition to the Container model, if you agree.