Define an OpenAPI Tag.
This macro will generate 1 implementation, [`Tag`].
## Example
```rust
use predawn::Tag;
/// This doc will be used as the tag description
#[derive(Tag)]
#[tag(rename = "This a tag")]
pub struct Hello;
```
`rename` is optional, default is the type name.
[`Tag`]: https://docs.rs/predawn/latest/predawn/trait.Tag.html