[Rework] Binary Cache: Add write-back support

[fran6co]

Trying to implement #1406 with the new changes in the code

[fran6co]

I'm not sure I like the writeback option and I think it makes more sense like the original PR to write-back on every write provider given that we expect the package to be uploaded on build and to be there later on, so a write source not having a package it's more about being cleared at some point and I think it's expected to be refilled if marked as write

[fran6co]

In any case, I think push_success should check if the package is there already to avoid uploading when it doesn't need to. I could add the check just on writeback, but is there a reason why it's set to overwrite on push_success even if the package is already there?