Mark existing LFS object for upload for forks

What does this MR do?

When source project already has the LFS object to be uploaded, it should still be linked to the fork project. That way, forks has access to their own LFS objects.

To achieve that, in this MR, we need to check if the object exists in the lfs_objects of the fork instead of all_lfs_objects so it can still be uploaded. For the download operation, it should be kept as is until we get to #122002 (closed).

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• [-] Tested in all supported browsers
• [-] Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

If this MR contains changes to processing or storing of credentials or tokens, authorization and authentication methods and other items described in the security review guidelines:

• [-] Label as security and @ mention @gitlab-com/gl-security/appsec
• [-] The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• [-] Security reports checked/validated by a reviewer from the AppSec team

Closes #207216 (closed)