Add conan cache backup-upload

[AbrilRBS]

Changelog: Feature: Add conan cache backup-upload to upload all the backup sources in the cache, regardless of which references they are from
Docs: conan-io/docs#3438

Took me a bit to find the better way to cleanly separate the backup file recolection api, but I like how simple it looks in the end.

Not sold on the api names nor command names, but those are minor details that can be changed before merging, the PR is more to see if we like this approach

Note that I tried to add an option akin to the --metdata flag in conan upload, but quickly found that it didn't feel a nice fit, so turned away from it

[memsharded]

I like it! it is indeed clean and nice.

Just a minor comment about the public API, if we wanted to avoid splitting it into 2, but not really opposed to current proposal.

[memsharded]

Amazing!