feat: implement ProtoMethods.CheckInitialized to avoid the slow path

CheckInitialized checks for missing required fields, which is not
relevant for a proto3 implementation such as Pulsar. However, without it
protobuf will fall back to a slow fallback that allocates memory per
serialization.

Before:

Benchmark_Pulsar_Marshal-6       7251019               153.9 ns/op              51.57 B/serial        96 B/op          2 allocs/op
Benchmark_Pulsar_Unmarshal-6     6409070               187.2 ns/op              51.63 B/serial       160 B/op          5 allocs/op

After:

Benchmark_Pulsar_Marshal-6     	12480471	        91.47 ns/op	        51.61 B/serial	     128 B/op	       2 allocs/op
Benchmark_Pulsar_Unmarshal-6   	10919157	       108.6 ns/op	        51.58 B/serial	     128 B/op	       3 allocs/op

Author: elias-orijtech

features/fastreflection/proto_message.go

@@ -306,15 +306,16 @@ func (g *fastGenerator) genProtoMethods() {
 	g.genSizeMethod()
 	g.genMarshalMethod()
 	g.genUnmarshalMethod()
+	g.genCheckInitializedMethod()
 
 	g.P(varName, " = &", protoifacePkg.Ident("Methods"), "{ ")
 	g.P("NoUnkeyedLiterals: struct{}{},")
 	g.P("Flags: ", protoifacePkg.Ident("SupportMarshalDeterministic"), "|", protoifacePkg.Ident("SupportUnmarshalDiscardUnknown"), ",")
 	g.P("Size: size,")
 	g.P("Marshal: marshal,")
 	g.P("Unmarshal: unmarshal,")
+	g.P("CheckInitialized: checkInitialized,")
 	g.P("Merge: nil,")
-	g.P("CheckInitialized: nil,")
 	g.P("}")
 	g.P("}")
 }

features/fastreflection/proto_unmarshal.go

@@ -126,6 +126,13 @@ func (g *fastGenerator) genUnmarshalMethod() {
 	g.P(`}`)
 }
 
+func (g *fastGenerator) genCheckInitializedMethod() {
+	// checkInitialized method.
+	g.P(`checkInitialized := func(input `, protoifacePkg.Ident("CheckInitializedInput"), `) (`, protoifacePkg.Ident("CheckInitializedOutput"), `, error) {`)
+	g.P(`return `, protoifacePkg.Ident("CheckInitializedOutput"), `{}, nil`)
+	g.P(`}`)
+}
+
 func (g *fastGenerator) decodeVarint(varName string, typName string) {
 	g.P(`for shift := uint(0); ; shift += 7 {`)
 	g.P(`if shift >= 64 {`)