Properly update nuget package file size

What does this MR do?

Nuget package uploads are done using the multi part format. As such, the size of the request is unknown during the upload.

When processing a nuget package, we should properly update the package file size attribute using the file size from the object storage.

This MR is part of the nuget MVC (See the epic &2271 (closed)). This MR is gated behind this feature flag (scoped by project): nuget_package_registry. This MR doesn't need documentation or a change log entry.

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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