The `WorkspacePlugin` is not a top-level entity. Yet, it extends
`BaseDomain`. It gets all the fields from `BaseDomain`, but since it
doesn't have a collection of its own, nothing is ever used.
This is fine today, with the `@Document` annotation, but with Postgres,
with `@Entity` annotation, querydsl is getting confused and is creating
a `QWorkspacePlugin` class, which is producing a compile error in
`QWorkspace.plugins` field.
This PR removes this unneeded inheritance.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit
- **Refactor**
- Updated the structure of the Workspace Plugin to enhance performance.
- **Bug Fixes**
- Ensured all plugins, including previously inaccessible ones, are now
correctly retrievable.
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