Consolidate upload preparers

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 for WG members understand the code better and help new contributors.

All preparer types are consolidated in the upload package so they are easier to find and show that when a new type is needed, this is where it can be added.

Currently, preparers are not mentioned in developer docs. A separate change will be created for it to guide contributors whether they need to add a new preparer or use an existing one.

Note

This is the 1st part of the 3 related MRs. To understand the changes further, see #351657 (comment 838111408).

MR acceptance checklist

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

Related to #351657 (closed)