Allow use of multiple accounts for prefilling

[GotenXiao]

An operator may want to cache a set of titles to prepare for a network/event, but not own all of them on one account; barring use of tools like Steam Family Sharing (which have limitations if the accounts are in use), there's currently no convenient way to allow multiple accounts to be used to download a desired list of app IDs.

lancache-autofill allowed authorising multiple accounts and then using the relevant accounts with ownership of a title to perform the download.

[frozenjava]

This feature would be really useful since my girlfriend and I have separate steam libraries that'd we'd both like cached. For now Im going to try just having duplicate configs and change it out for whichever account I'm wanting to use at the time