Refactoring: LFS objects fetching during the import

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Summary

All of the classes are inside Projects::LfsPointers module:

flowchart TB

LfsImportService#execute --> LfsObjectDownloadListService#each_list_item
LfsObjectDownloadListService#each_list_item --> LfsDownloadLinkListService#each_link
LfsDownloadLinkListService#each_link -->|lfs object link| LfsObjectDownloadListService#each_list_item
LfsObjectDownloadListService#each_list_item -->|lfs object link| LfsDownloadService#execute

The code inside classes more or less readable but connections between those classes and their responsibility getting into confusion. All we need is to re-struct this part of code. Maybe more horizontal structure like one organiser that calls each action-class sequentially. And do the single time context verification instead of few dublications as it currently works (for ex. project.lfs_enabled?)

Improvements

Increase readability & maintainability of the code

Involved components

Importer	How LFS are imported
GitHub	Uses Projects::LfsPointers::LfsObjectDownloadListService, which was modified by this MR
BitbucketServer	Uses Projects::LfsPointers::LfsImportService#execute, which the downstream code was modified by this MR
GitlabProject	Imports LFS objects contained in the imported .tar.gz using a different method
Projects::ImportService	Uses Projects::LfsPointers::LfsImportService#execute, which the downstream code was modified by this MR