#[soa_attr] on field

Sometimes we want to add attribute to generated struct's field.
For example:
```rust
#[derive(StructOfArray)]
#[soa_derive(Serialize, Deserialize)]
struct Point {
    x: f32,
    y: f32,
    // Skip serialization for PointVec::meta
    #[soa_attr(Vec, serde(skip))]
    meta: bool,
}
```


