Built-in flight plugin for additional bundler (#76014)
Adds support for a built-in Rspack Flight Client Entry Plugin implementation alongside the existing webpack version.
Ported from https://github.com/vercel/next.js/tree/wbinnssmith/try-ci-test, originally by @hardfist.
- Introduces a new `RspackFlightClientEntryPlugin` class that wraps Rspack's built-in Flight Client Entry Plugin
- Conditionally uses either the Rspack or webpack implementation based on environment flags
