In order to customize how to fields of native types can be accessed via
CEL, struct tag support has been added. It is now possible to provide
a struct field tag, that will act as the name for the corresponding
field

Here an example

```
type Person struct {
  Name string
  Age  int `cel:"age"`
}
```

and here is how to access the `Age` field from CEL:
```
person.age
```

Make struct tag parsing opt-in

Add docs