Minimal requirements for Open Podcast API ™ supporting servers & clients

[keunes]

Just like with 'Podcasting 2.0' specs, we can't expect all clients & servers to support all calls. I think it'd be good to specify which ones are minimally required in order to claim one supports the 'Open Podcast API' sync.

Inspired by this post: AntennaPod/AntennaPod#4960 (comment)

• it would need to use identifiers that can be retrieved from the feed instead of the internal IDs you are currently using (GUUID if available, otherwise maybe url). Also:
• Get the state + position of all played episodes at once using the API
• Get all episode/subscription changes after a specific date (otherwise, we need to request every single episode of every single subscription on each sync)