snapcast: add package snapserver and snapclient

snapcast: add package snapserver and snapclient

Snapcast [1] is a multiroom client-server audio player, where all
clients are time synchronized with the server to play perfectly synced
audio.

It's not a standalone player, but an extension that turns your existing
audio player into a Sonos-like multiroom solution.
Audio is captured by the server and routed to the connected clients.
Several players can feed audio to the server in parallel and clients can
be grouped to play the same audio stream.
One of the most generic ways to use Snapcast is in conjunction with the
music player daemon (MPD) [2] or Mopidy [3].

The inclusion starts with version 0.28.0 because of OpenSSL dependency
issues of version 0.29.0. Newer versions (e.g. 0.32.3, see [4])
with optional OpenSSL support, will be incorporated with flavours,
like "-mini" and "-full" in the upcoming PRs.
More about this topic please check the inclusion PR at [5]!

[1]: https://github.com/badaix/snapcast
[2]: http://www.musicpd.org/
[3]: https://www.mopidy.com/
[4]: https://github.com/badaix/snapcast/blob/develop/changelog.md
[5]: https://github.com/openwrt/packages/pull/23956#discussion_r2330463363

Fixes: #23924
Suggested-by: Tianling Shen <[email protected]>
Suggested-by: George Sapkin <[email protected]>
Signed-off-by: Szabolcs Hubai <[email protected]>