Support for generating Proto fields with 'optional' keyword

[chairbender]

I was looking for a way to be able to define the OpenAPI YAML in such a way that gnostic will generate protobuf with some fields using the "optional" keyword (which I think is kind of the reverse of what #275 was focused on). We want to use this optional keyword because we want to be able to check for existence vs. a field being set to its default value.

Is this currently possible, or is there a workaround to achieve this?