Signal proto3 optional fields supporting for 2.27

[Tim-Zhang]

It's a backport of #626

fixes #625

Tested this locally via cargo install --force --path . and then running against a protobuf definition containing an optional field via protoc --rust_out=. test.proto where test.proto contained:

syntax = "proto3";

message Test {
  optional string value = 1;
}

The generated code includes the following (note the optional field):

#[derive(PartialEq,Clone,Default)]
pub struct Test {
    // message oneof groups
    pub _value: ::std::option::Option<Test_oneof__value>,
    // special fields
    pub unknown_fields: ::protobuf::UnknownFields,
    pub cached_size: ::protobuf::CachedSize,
}

#[derive(Clone,PartialEq,Debug)]
pub enum Test_oneof__value {
    value(::std::string::String),
}

[Tim-Zhang]

@stepancheg may I bother you for a review on this patch?

[stepancheg]

Published 2.28.0.

Sorry for delay.

Crate is low maintenance, and looking for a new maintainer.