perf: added caching to XBlockSkillsViewSet list endpoint

[hamza-56]

PROD 4135

Merge checklist:

• Any new requirements are in the right place (do not manually modify the requirements/*.txt files)
  • make upgrade && make requirements have been run to regenerate requirements
• ./manage.py makemigrations has been run
  • Checkout the Database Migration Confluence page for helpful tips on creating migrations.
  • Note: This must be run if you modified any models.
    • It may or may not make a migration depending on exactly what you modified, but it should still be run.
• Version bumped
• Changelog record added

Post merge:

• Tag pushed and a new version released
  • Note: Assets will be added automatically. You just need to provide a tag (should match your version number) and title and description.
• After versioned build finishes in GitHub Actions, verify version has been pushed to PyPI
  • Each step in the release build has a condition flag that checks if the rest of the steps are done and if so will deploy to PyPi.
    (so basically once your build finishes, after maybe a minute you should see the new version in PyPi automatically (on refresh))
• PR created in course-discovery to upgrade dependencies (including taxonomy-connector)
  • This must be done after the version is visible in PyPi as make upgrade in course-discovery will look for the latest version in PyPi.

[Ali-D-Akbar]

Looks good to me.

[jajjibhai008]

Could you please add the necessary details along with the ticket link in the PR description? Also, try to resolve the coverage issues before merging.
Other than that, everything looks good to me 👍.

[DawoudSheraz]

We should add a unit test for this.