decoder/plugin: add method protocols()

Similar to commit 4e2a551f30 but for
decoder plugins.  This is tailored for the FFmpeg decoder plugin which
implements some protocols (e.g. RTSP) as demuxer plugin.

test/run_decoder.cxx

@@ -204,7 +204,12 @@ try {
 	}
 
 	MyDecoderClient client(c.seek_where);
-	if (plugin->file_decode != nullptr) {
+	if (plugin->SupportsUri(c.uri)) {
+		try {
+			plugin->UriDecode(client, c.uri);
+		} catch (StopDecoder) {
+		}
+	} else if (plugin->file_decode != nullptr) {
 		try {
 			plugin->FileDecode(client, FromNarrowPath(c.uri));
 		} catch (StopDecoder) {