`openapiv3`: Map explicit field presence to `null` type

Feature

Map proto2, proto3, and editions field presense to openapi v3 null types.

https://learn.openapis.org/upgrading/v3.0-to-v3.1.html#replace-nullable-with-type-arrays

https://protobuf.dev/programming-guides/field_presence/

I think fields with explicit presence should have null in their type array.

For example

proto3 string - implicit presense - type: ["string"] proto3 optional string - explicit presense - type: ["string", "null"]

@JackBogart had some input on this in the PR.