Accept v2 infohashes for download

[sledgehammer999]

Partially fixes #15287

I am also working on hiding the padding files but the solution is a bit involved. So I made this PR as standalone.

[glassez]

I am also working on hiding the padding files but the solution is a bit involved.

IMO, we need to completely hide these files behind an abstraction layer. For example, we can keep "file indexes" mapping, so that only the indexes of real files are visible from the outside (this also applies to the visible number of files).
I can do it if you want.

[sledgehammer999]

I can do it if you want.

I'll have a draft ready in a few hours. Then we can talk.

[sledgehammer999]

@glassez You can go ahead in implementing the hiding of pad files. I went to implement the webapi portion but I see it keeps record of the files differenlty from the GUI part. And I would need to do more extensive internal refactoring. So it is best that you do it because it'll much more time with me.

[sledgehammer999]

@glassez a hint: Litbtorrent file_storage has a function that tells us if a certain file is a pad file. You don't need to reinvent the wheel.