Enhancement: Add filter criterion for subscription state

### What problem does this address?

I'm copying the issue from Automattic/pocket-casts-ios#438 opened by @aidano since it's the exact request I have and already has an associated pull request.

> I'd like to show, for example, any Downloaded, In Progress episodes irrespective of subscription state.
>
> Proposal: Create a new filter that allows for podcast subscription state to be defined ( All | Subscribed | Not Subscribed)
>
> Since current behavior is that a filter automatically includes only episodes of subscribed podcasts, perhaps the above criterion can default to Subscribed when presented in the UI so that existing filters remain backwards compatible, and are updated only if the user changes the value to include All or Not Subscribed.

This would resolve the unexpected (to me) behavior that a filter showing "Downloads" does not actually include all downloads but only downloads from subscriptions.

### What is your proposed solution?

Add a new filter state to account for subscription state so that users can create filters for "All Downloads" instead of just "Downloads from subscribed podcasts".

I would actually be interested even in a filter of "Downloads from Unsubscribed Podcasts" which would also be possible with the proposed change.

### Did you search for existing feature suggestions?

- [X] I have searched for existing feature suggestions.

Start by tracing how Android filters represent subscription state and how the existing Downloads criterion limits results; the issue does not name files or tests. Review the linked iOS request for context. Done means filters can distinguish All, Subscribed, and Not Subscribed while preserving existing subscribed-only behavior by default.

Written by the indexing model from the issue text.