Derive macro for fieldmasker::MaskSpec.

Features

• Structs with named fields: builds an Object schema using field names, honoring Serde field attributes.
• Newtype tuple structs: delegate to the inner type’s schema.
• Other tuple structs, enums, and unions: treated as scalar.

Serde attributes honored

• #[serde(rename = "...")]
• #[serde(rename(serialize = "..."))] (preferred when present)
• #[serde(rename_all = "...")] on the container
• #[serde(skip)] / #[serde(skip_serializing)]

Unsupported (for now)

• #[serde(flatten)] — emits a compile error. Support can be added once the core crate exposes object-schema accessors at runtime.