feat: add endpoints for new siteEnrollment config property | LLMO-421

[JayKid]

Please ensure your pull request adheres to the following guidelines:

• make sure to link the related issues in this description. Or if there's no issue created, make sure you
  describe here the problem you're solving.
• when merging / squashing, make sure the fixed issue references are visible in the commits, for easy compilation of release notes

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• make sure you add a "Not implemented yet" note the endpoint description, if the implementation is not ready
  yet. Ideally, return a 501 status code with a message explaining the feature is not implemented yet.
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Related Issues

• LLMO-421 | Backend for user/site/org configurations

Related PRs

• feat: add new site config map<string, string> property | LLMO-421 spacecat-shared#957

Relies on the above for a new siteEnrollment property called config (Map<string,string>)

[github-actions]

This PR will trigger a minor release when merged.