[Feature request] [protobuf] add optional keyword

[w01fgang]

Clear and concise description of the problem

There's no way to add optional keyword for protobuf. The optional keyword behaviour is not always the same and depend from the client language as well. For example Go client has an option that defines the optional keyword behaviour. So this can be solved only with decorators because there's no constant connection between the open api optional filed definition and protobuf optional keyword presence.

In this definition User has ordersCount that required in the api response and must have optional keyword to set default value to 0:

model User {
  @field(1)
  id: string;

  @field(10)
  address?: Address;

  @optional // this is not implemented yet
  @field(11)
  ordersCount: int32 = 0;
}

Checklist

• [X] Follow our Code of Conduct
• [X] Read the docs.
• [X] Check that there isn't already an issue that request the same feature to avoid creating a duplicate.

[markcowl]

investigation