Fix missing `forked_from_project` property for unauthenticated users

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What does this MR do and why?

Fix #361952: missing forked_from_project property in projects API for unauthenticated users

The API response returned to unauthenticated users is based on BasicProjectDetails, which is the same class as the one used in projects list.

This introduces a new class, between BasicProjectDetails and Projects in the class hierarchy, that provides this property.

This replaces !152231 (closed) with a more limited scope.

Submitted as part of gitlab-org/developer-relations/contributor-success/team-task#479

Changelog: fixed

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