Support feature-gated rust attributes

Consider e.g.:
```
"attr.rust.derive": "::serde::Serialize, ::serde::Deserialize",
```

This should only be active if the `serde` feature is enabled, i.e.
```rust
#[cfg_attr(feature = "serde", derive("::serde::Serialize, ::serde::Deserialize"))]
```
rather than merely
```rust
#[derive("::serde::Serialize, ::serde::Deserialize")]
```