feat: add endpoint to get draft version of a library component asset

[Ian2012]

Description

Creates a new to get the draft version of a library component asset via its usage_key and asset path.

Supporting information

Needed for openedx/frontend-app-authoring#1403

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Thanks for the pull request, @Ian2012!

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2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

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2U Release Notice: This PR has been deployed to the edX production environment.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX staging environment in preparation for a release to production.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.

[edx-pipeline-bot]

2U Release Notice: This PR has been deployed to the edX production environment.