Support for podcast:recommendations tag

# Checklist

- [x ] I have used the search function to see if someone else has already submitted the same feature request.
- [x ] I will only create one feature request per issue.
- [x ] I will describe the problem with as much detail as possible.

# System info

**App version**: 2.3.0-beta2

**App source**: Google Play

# Feature description

**Problem you may be having, or feature you want**:

Podcasters often mention tips and recommendations in their content. Sometimes they list them in the shownotes, but they're not always easy to find. I would love to have their recommendations in a clear overview in my podcast app to quickly find them.

**Suggested solution**:

Parse recommendations as provided via the Recommendations tag of the Podcast Namespace. List the recommendations at the bottom of the Shownotes, with horizontal scrolling or a grid (2 columns, rows as necessary).

Note that this tag has not been formally adopted, yet. If there is interest from our side to implement this, we should chip in at the Podcast Namespace discussion. Implementation by one hoster and one client is required for official adoption. We could fulfil the client requirement :)

If the recommended content type is `podcast`, I'd want to be able to have a quick view of the podcast (same as when now you would preview a podcast before adding it). Ideally, if the type is `episode` then ask me what to do (play now, add to queue)¹. If the recommended content is of any other type, just open the link in the default browser.

¹ I realise this is complex as currently, just like previews, single episodes cannot be dealt with. Maybe we can find a solution, making use of the Podcast tags. If I play a single episode, add it with the tag Not-subscribed, which is hidden from the main list, not kept up to date, etc. See also #4710.

**Screenshots / Drawings / Technical details**:

* [Mock-up](https://design.penpot.app/#/view/e357a320-7535-11eb-8a88-eb0147e048f5?page-id=c3a9e280-89c7-11eb-831c-a15b28ff763c&index=0&share-id=e321ee60-7002-11ec-8da6-c2b142838c8b)
* [Tag documentation](https://github.com/Podcastindex-org/podcast-namespace/blob/main/proposal-docs/recommendations/recommendations.md)
* [GitHub issue requesting introduction of this namespace tag](https://github.com/Podcastindex-org/podcast-namespace/issues/205)

I propose this gets loaded into the database when a) playback is started (if network is available) and b) episode is downloaded. When playback finished or episode is deleted, this data can be removed from the database to save space. This would follow the same mechanism as for [Transcripts](https://github.com/AntennaPod/AntennaPod/issues/4935).

As this is an external file and podcasters may expand their recommendations after release of the episode, this info should be updated when playback (re)starts (whenever possible, i.e. if network connection is available).