fix: Swift 6.3: Explicit dependency on swift-nio for NIOFoundationCompat

This PR is an attempt to fix what (seems to be) a recent break I see when building on Linux under Swift 6.3. `stripe-kit` imports `NIOFoundationCompat` in its code but doesn't declare a dependency on it in Package.swift. Looking upstream, the `AsyncHTTPClient` target in `async-http-client` comprehensively depends on the other products of `swift-nio` but it doesn't depend on `NIOFoundationCompat` (only the `AsyncHTTPClientTests` target does): https://github.com/swift-server/async-http-client/blob/main/Package.swift Somehow previously this worked, but in Swift 6.3 the behaviour of SPM seems have changed. From my testing, I expect the api-breakage test on this PR to fail, but I'm unsure if this is a legitimate failure, sorry.