Add support for private uploads / presigned URLs

Here is a reroll of the patch in [2772847-59] against 2.0.x HEAD.

Do not use the patch in 62, it contains errors. Reroll of the reroll incoming.

Note I've discussed this with @deviantintegral and they no longer have a use for this patch. This issue may be closed once I have no further need for it unless anyone else comments that they still require it.

Needs reroll to apply against 2.1.x following [#3409513]

Apologies, my PHPStorm settings have mangled the formatting in that push. Will rectify forthwith...

For some reason* the patch generated on gitlab for the current state of this merge request won't apply against current 2.1.x, but a manually generated diff between the merge request branch and 2.1.x will. This is v odd especially as the MR will merge without conflicts.

So here's a patch.

*probably because I somehow did something wrong merging in the changes commit in 📌 Fix PHPCS and PHPStan issues reported by Gitlab CI Fixed 🤷‍♀️