Add support for proto3 `optional` keyword

### 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](https://github.com/protocolbuffers/protobuf/releases/tag/v3.15.0), documented [on this page](https://protobuf.dev/programming-guides/proto3/#field-labels).

### 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`.