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/HybridDsdDecoderPlugin.cxx

@@ -266,17 +266,9 @@ static const char *const hybrid_dsd_suffixes[] = {
 	nullptr
 };
 
-const struct DecoderPlugin hybrid_dsd_decoder_plugin = {
-	"hybrid_dsd",
-	InitHybridDsdDecoder,
-	nullptr,
-	HybridDsdDecode,
-	nullptr,
-	nullptr,
+constexpr DecoderPlugin hybrid_dsd_decoder_plugin =
 	/* no scan method here; the FFmpeg plugin will do that for us,
 	   and we only do the decoding */
-	nullptr,
-	nullptr,
-	hybrid_dsd_suffixes,
-	nullptr,
-};
+	DecoderPlugin("hybrid_dsd", HybridDsdDecode, nullptr)
+	.WithInit(InitHybridDsdDecoder)
+	.WithSuffixes(hybrid_dsd_suffixes);