smartplaylist: allow exporting item fields, rename m3u8 output to extm3u

[mgoltzsche]

Description

Allow generating extm3u playlists so that they contain additional item fields such as the id.

The feature is required by the webm3u plugin (M3U server) to transform playlists using a request based item URI template which may require additional fields such as the id, e.g. beets:library:track;$id.

To Do

• Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)
• Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• Tests. (Very much encouraged but not strictly required.)

[Serene-Arc]

Thank you for this PR! The code seems fine but might I ask that you add some more to the docs? Perhaps the keys to the attributes most likely to be used and an explanation for when they might be required, such as the web service you specified.

[JOJ0]

Hi @mgoltzsche, great addition to smartplaylist and very interesting plugin you wrote. Please also add a link to your plugin's repo to this section of the docs when you are coming back to working on this PR: https://beets.readthedocs.io/en/latest/plugins/index.html#interoperability
Just including the docs change within this PR is fine in my opinion.

[mgoltzsche]

I rebased the PR now to resolve the merge conflict, added an entry to the interoperability section of the plugins/index.rst, added more info to the smartplaylist.rst.
Also, I changed the option name from attributes to fields because it is shorter and more aligned with the existing CLI terminology (beet fields).
Unfortunately, the GitHub data of the PR seems to be inconsistent since it doesn't show my force-pushed commit 😞....

[mgoltzsche]

Okay, as described here re-selecting beetbox:master as base branch within the GitHub GUI updates the PR and triggers CI but now the diff also includes all other commits on the master branch since this PR was created 😞. However, my changes are still within a single commit which you could re-review.

[mgoltzsche]

@JOJ0 are you okay with having the webm3u plugin link within the "Other plugins" section of the documentation as opposed to putting it in the interoperability section? Otherwise, do you think the webrouter plugin should also be listed in the interoperability documentation section?

[JOJ0]

Ah yes please put everything into the Other Plugins section. Seems like we file all of externally hosted plugins there. My suggestion here was wrong. I'm sorry: #5121 (comment)

[mgoltzsche]

Rebased to resolve merge conflict within changelog.

[JOJ0]

Sorry for the massive delay in reviewing, I wanted to do this earlier. Code is fine, there is only one thing that I suggest to improve: Please add one or two example lines to the docs. How will a playlist entry look like with a specific config?

[JOJ0]

Something like this: Add a config example and the resulting line on the playlist file to the docs.

[mgoltzsche]

Okay, I added an example configuration and extm3u playlist to the documentation now. Please re-review!

[mgoltzsche]

After some problems with the docs linter now the PR is ready for review again.
Also, I made the documentation mention the autogenre plugin.

[JOJ0]

BTW I like the tiny PlaylistItem class, something like that could also help with my never finished project of providing a general m3u handling solution in beets. Currently it's rather unspectacular, it has save/load functionality only: https://github.com/beetbox/beets/blob/master/beets/util/m3u.py

oops oof-topic alert :-)