Add support for Protobuf 3.15 optional fields

[kelteseth]

Protobuf 3.15 (Feb 18, 2021) introduced optional messages to know if a message value was set or not.

Example from: https://stackoverflow.com/a/62566052/12619313

syntax = "proto3";

message Foo {
    int32 bar = 1;
    optional int32 baz = 2;
}

A has_baz()/hasBaz() method is generated for the optional field above, just as it was in proto2.

This is useful for example if you want to display imported 3d OpenStreetMap data that only sometimes contains height data.

message Vec3 {
  float x = 1;
  optional float y = 2;  // Godot UP
  float z = 3;
}