Generate enum with values when using protoc-gen-openapi

Thanks for this helpful tool.

I come across this issue when I'm using protoc-gen-openapi plugin to generate OpenAPI3 specification from protobuf definition.

I have a State type and fields of State type are marked with `enum` format.

```protobuf
enum State {
state1 = 0;
state2 = 1;
state3 = 2;
}
```

```yaml
state:
type: integer
format: enum
```

However, for `enum` in OpenAPI3 should be

```yaml
state:
type: integer
enum:
- 0
- 1
- 2
```

Is there any feature that we can configure to generate `enum` schema?