Multi-queue functionality

[linuxgoose]

Description

Multi-queue functionality. Closes #2.

Checklist

• [ x ] I have read the contribution guidelines: https://github.com/AntennaPod/AntennaPod/blob/develop/CONTRIBUTING.md#submit-a-pull-request
• [ x ] I have performed a self-review of my code, going through my changes line by line and carefully considering why this line change is necessary
• [ x ] I have run the automated code checks using ./gradlew checkstyle lint
• [ x ] My code follows the style guidelines of the AntennaPod project: https://antennapod.org/contribute/develop/app/code-style
• [ x ] I have mentioned the corresponding issue and the relevant keyword (e.g., "Closes: #xy") in the description (see https://docs.github.com/en/issues/tracking-your-work-with-issues/linking-a-pull-request-to-an-issue)
• [ x ] If it is a core feature, I have added automated tests