In https://gitlab.com/gitlab-org/gitlab/-/issues/351657, 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.
- Renamed `SaveFileOpts` to `UploadOpts` to be consistent with language.
- Removed `SaveFileFromDisk` as it's not being used at all.