Embeddable Once cache SDK with signed XCFramework release

Once now ships an embeddable cache SDK for Rust, Swift, Objective-C, and other languages via a stable C ABI. You can add the once crate in Rust to read and write blobs and action results from the local cache, or drop the bundled Once.xcframework.zip into an Apple project and call the Swift OnceCache API. Each release is built, signed, notarized, and published as a release asset automatically.