Add support for proto3 optional keyword

[david-bakin-sl]

Problem

I'm always frustrated when I see google.protobuf.Int32Value in a protobuf schema to represent an optional int32 field, when the proto3 language has support for that directly via the optional keyword - including the programmer being able to detect whether the field is there or not. (As opposed to being "implicitly optional" in the usual proto3 way.)

This was added as a standard feature in protobuf v3.15.0, documented on this page.

Solution

Support optional in PBJ, hopefully identically to protoc from Google, but in any event meeting whatever (backward) compatibility we need w.r.t. our permanent protobuf structures.

Alternatives

Continue using wrappers such as google.protobuf.Int32Value. People who see that in our schemas will just understand it means optional int32.