Refactor upload package

What does this MR do and why?

In #351657 (closed), the goal is to document and refactor workhorse code related to object storage upload. This is to improve readability and help new contributors.

This include the following changes:

• Moved UploadArtifacts to upload package to consolidate all uploaders in a single package. Renamed it to Artifacts to be consistent with other uploaders.
• Unexported InterceptMultipartFiles to reduce the number of exported functions in upload package.

These upload middlewares are not yet mentioned in developer docs. A separate change will be made to add information about these uploaders and which one to use.

Note

This is the 2nd part of the 3 related MRs. Dependent on !80271 (merged). To understand the changes further, see #351657 (comment 838111408).

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #351657 (closed)