Refactor upload destinations

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:

• Created destination package to consolidate upload destination related logic. Used this name so we're consistent with the language we use for upload destination.
• Moved objectstore and filestore packages under destination package since these are the store used as destination for object storage upload.
• Made filestore package specific for storing to disk. This is to clarify that filestore package is responsible for uploading to disk instead of its previous responsibilities.
• Renamed SaveFileFromReader to Upload to be consistent with the language of object storage upload. The previous name is a bit too tied to implementation details.
• Removed SaveFileFromDisk as it's not being used at all.

Note

This is the 3rd part of the 3 related MRs. Dependent on !80281 (merged). To understand the changes further, see #351657 (comment 838111408).

MR acceptance checklist

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Related to #351657 (closed)