[Feature] support async download features

[kevincheng2]

Motivation

support async download features
cp from #4910

1. 将features的下载从 worker进程转移到 engine进程，避免gpu等待features下载，浪费算力。
2. engine 调度时增加异步等待机制，未完成异步处理的请求不下发至worker(支持多个异步事件，后续可以扩展其他异步任务)。
3. 取消features下载检查逻辑，下载发生错误时可以直接返回错误信息。

Modifications

将features下载由worker转移到engine中

Usage or Command

启动时需要增加参数：

  python -m fastdeploy.entrypoints.openai.api_server \
       ...
       --enable-async-download-features

性能测试后，删除该启动参数，改为默认打开

Accuracy Tests

Checklist

• Add at least a tag in the PR title.
  • Tag list: [[FDConfig],[APIServer],[Engine], [Scheduler], [PD Disaggregation], [Executor], [Graph Optimization], [Speculative Decoding], [RL], [Models], [Quantization], [Loader], [OP], [KVCache], [DataProcessor], [BugFix], [Docs], [CI], [Optimization], [Feature], [Benchmark], [Others], [XPU], [HPU], [GCU], [DCU], [Iluvatar], [Metax]]
  • You can add new tags based on the PR content, but the semantics must be clear.
• Format your code, run pre-commit before commit.
• Add unit tests. Please write the reason in this PR if no unit tests.
• Provide accuracy results.
• If the current PR is submitting to the release branch, make sure the PR has been submitted to the develop branch, then cherry-pick it to the release branch with the [Cherry-Pick] PR tag.

[paddle-bot]

Thanks for your contribution!