Support optional fields

No changes are required beyond setting this flag as twirphp doesn't
interact with message types at all.

Fixes:

    example.proto: is a proto3 file that contains optional fields, but code generator protoc-gen-twirp_php hasn't been updated to support optional fields in proto3. Please ask the owner of this code generator to support proto3 optional.

CHANGELOG.md

@@ -8,6 +8,9 @@ and this project adheres to [Semantic Versioning](http://semver.org/spec/v2.0.0.
 
 
 ## [Unreleased]
+### Added
+
+- Support for proto3 field presence (optional fields)
 
 
 ## [0.7.4] - 2021-07-31

protoc-gen-twirp_php/internal/gen/generator.go

@@ -30,6 +30,8 @@ type globalFileData struct {
 
 // Generate is the main code generator.
 func Generate(plugin *protogen.Plugin, version string) error {
+	plugin.SupportedFeatures = uint64(pluginpb.CodeGeneratorResponse_FEATURE_PROTO3_OPTIONAL)
+
 	namespaces := map[string]bool{}
 
 	for _, file := range plugin.Files {