Migrate the server-side live downloading to the new biliup::downloader::live API and simplify the actor model. Key changes:
- Introduce a new server/core/live module that builds LiveRequest/LiveOptions/LiveCredentials, maps runtimes, and creates danmaku clients.
- Replace legacy plugin/DownloadBase actor flow with LivePlugin-based workflows and a start_download_workflow entrypoint; monitor now acquires a Semaphore permit before starting a download to enforce concurrency.
- Remove the old plugin module and per-platform plugin plumbing; many platform implementations were moved/renamed under crates/biliup/src/downloader/live.
- Adjust DownloadManager, Monitor, and common download/upload code to use LivePlugin, LiveStream and related types; pass uploader channel and download slot semaphore into Monitor.
- Add download_stream helper to support direct FLV/HLS downloads via StatelessClient, and extend Streamlink downloader to support Niconico credentials/options.
- Update usage of context/streamer info and danmaku initialization to the new LiveStream-based model.
- Bump workspace version to 1.2.0 (Cargo.toml) and related package versions.
This refactor centralizes live logic on the shared biliup downloader live API, simplifies concurrency control, and removes the previous actor-based plugin indirection.
ForgQi