feat(cli): add configurable backend image fallback tags via CLI options (#8817)

* feat(cli): add configurable backend image fallback tags via CLI options

- Add three new CLI flags: --backend-images-release-tag, --backend-images-branch-tag, --backend-dev-suffix
- Add corresponding fields to SystemState for passing configuration
- Add WithBackendImagesReleaseTag, WithBackendImagesBranchTag, WithBackendDevSuffix options
- Modify getFallbackTagValues to use SystemState instead of environment variables
- Pass CLI options through to SystemState in run.go

Signed-off-by: localai-bot <localai-bot@users.noreply.github.com>

* fix: add missing os import in core/gallery/backends.go

Signed-off-by: localai-bot <localai-bot@users.noreply.github.com>

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Signed-off-by: localai-bot <localai-bot@users.noreply.github.com>
Co-authored-by: localai-bot <localai-bot@users.noreply.github.com>