Add LRCLIB as a provider for the lyrics plugin

[mumumumu]

Description

This PR adds support for LRCLIB as a lyrics provider.

I recently set up Navidrome on my home server and the web UI for that only supports embedded, synced lyrics. From what I can tell, the current lyric providers only return plain lyrics. I did a little research and found LRCLIB, which offers an easy to use API that includes both synced and plain lyrics. Note that all four parameters (track, artist, album, duration) to the /get endpoint are required.

Inspired by tranxuanthang/lrcget#2 (comment).

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.)

[sampsyo]

Wow; this looks really great! I've reviewed the code carefully and everything looks good from here.

The one high-level question I have is: Do we want to have this source enabled by default, given that it returns synced lyrics by default? To be specific, "synced" lyrics from LRCLIB look like this:

[00:09.17] Come back from San Francisco
[00:12.97] It can't be all that pretty
[00:16.75] When all of New York City misses you
...

…which may be surprising to most users of the plugin, who (from other sources) are expecting plain "unsynced" lyrics.

One option could be to add a new top-level configuration option to the plugin, synced. This would prefer synced lyrics for any source that provides them (currently just this new source), and otherwise would only try to get plain lyrics. Would this make sense?

[mumumumu]

@sampsyo thanks for the quick review, and that makes total sense. Added a synced option and updated the plugin docs.

[sampsyo]

Wonderful; this looks great! Thanks again for the awesome work on this.