feat: add support for public/private playlists in NSP import

Signed-off-by: Deluan <deluan@navidrome.org>

tests/fixtures/playlists/private_playlist.nsp

@@ -0,0 +1,11 @@
+{
+  "name": "Private Playlist",
+  "comment": "A smart playlist that is explicitly private",
+  "public": false,
+  "all": [
+    {"is": {"loved": true}}
+  ],
+  "sort": "title",
+  "order": "asc",
+  "limit": 100
+}

tests/fixtures/playlists/public_playlist.nsp

@@ -0,0 +1,11 @@
+{
+  "name": "Public Playlist",
+  "comment": "A smart playlist that is public",
+  "public": true,
+  "all": [
+    {"inTheLast": {"lastPlayed": 30}}
+  ],
+  "sort": "lastPlayed",
+  "order": "desc",
+  "limit": 50
+}