Visibility

Hi!

Thanks for the work so far!

Are there ways to omit methods and fields during OpenAPI3 generation based on [`google.api.field_visibility`](https://github.com/googleapis/googleapis/blob/master/google/api/visibility.proto) or any plans to add support?

Example:
```protobuf
service Example {
// Treated as public.
rpc PublicExample(GetExampleRequest) returns (GetExampleResponse);

// Limited to internal.
rpc InternalExample(GetExampleRequest) returns (GetExampleResponse) {
option (google.api.method_visibility).restriction = "INTERNAL";
}
}

message GetExampleRequest {
string id = 1; // Treated as public.
string internal_val = 2 [(google.api.field_visibility).restriction = "INTERNAL"]; // Internal-only.
}
```

It would be nice to have a `selector` parameter during selection where `PUBLIC`, `INTERNAL`, etc. could be set as well, but simply hiding `INTERNAL` fields in the generation would be helpful as well for folks providing public APIs and documentation.

Would PRs be considered in this regard?

Regards,
– Dani