Support proto3 optional fields

[APatrickJ]

Good day and thank you for providing this useful plugin.

Is it feasible to add support for proto3 optionals fields? When I attempt to use this tool on my proto3 files, I get the following error:

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