api/bilibili: support download episode in collection

[charles7668]

Previously, Cobalt only downloaded the first video in a Bilibili collection.
This PR adds the ability to download different episodes from the collection.

[wukko]

thank you for the pr! there are some changes that have to be made before we can merge this. you can just commit my suggestions.

i'd also make sure that the episode id is not a random string but is indeed an episode number. would also be nice to add episode tests to util/tests/bilibili.json

[charles7668]

thank you for the pr! there are some changes that have to be made before we can merge this. you can just commit my suggestions.

i'd also make sure that the episode id is not a random string but is indeed an episode number. would also be nice to add episode tests to util/tests/bilibili.json

If p is a random string, it will download the first episode. In bilibili.json, every p will be marked as successfully downloaded (even if p doesn't exist, it still downloads the first episode).
How can I add a test for this behavior?