Add support for groups

Hey,
would be great if we could support groups to simpify generating two OpenApis for example when you need API Versioning.
Maybe with a syntax like that:
```rust
#[utoipauto::group("v1")
...

#[utoipauto::group("v1, v2")
```

```rust
#[utoipauto(groups ="v1, v2"]
#[derive(OpenApi)]
#[openapi()]
pub struct ApiDocs;
```
