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/PcmDecoderPlugin.cxx

@@ -251,15 +251,6 @@ static const char *const pcm_mime_types[] = {
 	nullptr
 };
 
-const struct DecoderPlugin pcm_decoder_plugin = {
-	"pcm",
-	nullptr,
-	nullptr,
-	pcm_stream_decode,
-	nullptr,
-	nullptr,
-	nullptr,
-	nullptr,
-	nullptr,
-	pcm_mime_types,
-};
+constexpr DecoderPlugin pcm_decoder_plugin =
+	DecoderPlugin("pcm", pcm_stream_decode, nullptr)
+	.WithMimeTypes(pcm_mime_types);