smartplaylist: add --uri-template option

Beets web API already allows remote players to access audio files but it doesn't provide a way to expose the playlists defined using the smartplaylist plugin.
Now the smartplaylist plugin provides an option to generate ID-based item URIs/URLs instead of paths.
Once playlists are generated this way, they can be served using a regular HTTP server such as nginx.
To support integrations with more APIs than just the beets web API (e.g. AURA API), the new option has been defined as a template with an `$id` placeholder (assuming each integration requires a different path schema but they all rely on using the beets item `id` as identifier/path segment).

Example linking to beets web API: `beet splupdate --uri-template 'http://beets:8337/item/$id/file'`

(While it was already possible to generate playlists with URLs previously using the `prefix` option, it did not allow to generate ID-based URLs pointing to the beets web API but required to expose the audio files using a web server directly and refer to them using their file system path.)

Relates to beetbox#5037

beetsplug/smartplaylist.py

@@ -45,6 +45,7 @@ def __init__(self):
                 "playlist_dir": ".",
                 "auto": True,
                 "playlists": [],
+                "uri_template": None,
                 "forward_slash": False,
                 "prefix": "",
                 "urlencode": False,
@@ -72,6 +73,13 @@ def commands(self):
             action="store_true",
             help="display query results but don't write playlist files.",
         )
+        spl_update.parser.add_option(
+            "--uri-template",
+            dest="uri_template",
+            metavar="TPL",
+            type="string",
+            help="playlist item URI template, e.g. http://beets/item/$id/file.",
+        )
         spl_update.parser.add_option(
             "--extm3u",
             action="store_true",
@@ -208,6 +216,7 @@ def update_playlists(self, lib, extm3u=None, pretend=False):
                 "Updating {0} smart playlists...", len(self._matched_playlists)
             )
 
+        tpl = self.config["uri_template"].get()
         playlist_dir = self.config["playlist_dir"].as_filename()
         playlist_dir = bytestring_path(playlist_dir)
         relative_to = self.config["relative_to"].get()
@@ -239,8 +248,13 @@ def update_playlists(self, lib, extm3u=None, pretend=False):
                 if m3u_name not in m3us:
                     m3us[m3u_name] = []
                 item_path = item.path
-                if relative_to:
-                    item_path = os.path.relpath(item.path, relative_to)
+                if tpl:
+                    item_path = tpl.replace("$id", str(item.id)).encode("utf-8")
+                else:
+                    if relative_to:
+                        item_path = os.path.relpath(item.path, relative_to)
+                    item_path = prefix + item_path
+
                 if item_path not in m3us[m3u_name]:
                     m3us[m3u_name].append({"item": item, "path": item_path})
                     if pretend and self.config["pretend_paths"]:
@@ -272,7 +286,7 @@ def update_playlists(self, lib, extm3u=None, pretend=False):
                             comment = "#EXTINF:{},{} - {}\n".format(
                                 int(item.length), item.artist, item.title
                             )
-                        f.write(comment.encode("utf-8") + prefix + path + b"\n")
+                        f.write(comment.encode("utf-8") + path + b"\n")
             # Send an event when playlists were updated.
             send_event("smartplaylist_update")