Allow users to configure the container image used for init containers when creating a workspace
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## Description
When creating a workspace, we use a container image(e.g. `registry.gitlab.com/gitlab-org/workspaces/gitlab-workspaces-tools:9.0.0`) to inject various tools inside the container where we want to start the IDE.
As a user(administrator), I want a way to configure this image to ensure Workspaces works properly is an isolated/offline environment.
Duplicate issues
- https://gitlab.com/gitlab-org/gitlab/-/issues/507857#note_2452111825
- https://gitlab.com/gitlab-org/gitlab/-/issues/508162+
## Acceptance criteria
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- [ ] [Describe what must be achieved to complete this issue.]
- [ ] [If applicable, please provide design specifications for this feature/enhancement.]
- [ ] [If applicable, please list any technical requirements (performance, security, database, etc.)]
## Implementation plan
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[Provide a high-level plan for implementation of this issue, including relevant technical and/or design details.]
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