Downloads: Add Prioritized queue system for Downloads

[bjtitus]

Description

All downloads are currently queued on background URLSessions which provide nothing in terms of order or prioritization. Ideally, we would prioritize some episodes over others (most recent, top of Up Next, etc.) which users are most likely to need access to.

There are several requests to limit parallel downloads (https://github.com/Automattic/pocket-casts-ios/issues/1520, https://github.com/Automattic/pocket-casts-ios/issues/1049) which could be used here to limit our internal query for users who experience problems with downloads.

Step-by-step reproduction instructions

1. Set up Auto Downloads on a filter with many episodes
2. Use a slower network connection
3. Notice that downloads may occur in any order

Screenshots or screen recording

No response

Did you search for existing bug reports?

• [X] I have searched for existing bug reports.

Device, Operating system, and Pocket Casts app version

No response