decoder/Plugin: simplify compile-time initialization

Add a `constexpr` constructor and several `constexpr` methods to
construct a DecoderPlugin at compile time, in a way which allows
adding new methods later without having to edit each plugin.

src/decoder/plugins/OpusDecoderPlugin.cxx

@@ -384,15 +384,8 @@ static const char *const opus_mime_types[] = {
 
 } /* anonymous namespace */
 
-const struct DecoderPlugin opus_decoder_plugin = {
-	"opus",
-	mpd_opus_init,
-	nullptr,
-	mpd_opus_stream_decode,
-	nullptr,
-	nullptr,
-	mpd_opus_scan_stream,
-	nullptr,
-	opus_suffixes,
-	opus_mime_types,
-};
+constexpr DecoderPlugin opus_decoder_plugin =
+	DecoderPlugin("opus", mpd_opus_stream_decode, mpd_opus_scan_stream)
+	.WithInit(mpd_opus_init)
+	.WithSuffixes(opus_suffixes)
+	.WithMimeTypes(opus_mime_types);