Non-blocking Activity task implementations

Issue

The TaskActivity interface is currently has a synchronous run method that returns that output of that activity:

public interface TaskActivity {
    Object run(TaskActivityContext ctx);
}

There's currently no way to define a non-blocking, asynchronous activity implementation. This can result in issues related to the scalability of applications if many activity tasks need to be executed concurrently.

Proposal

There should be an alternative mechanism for implementing non-blocking, asynchronous activity tasks. For example:

public interface AsyncTaskActivity {
    CompletionStage<Object> run(TaskActivityContext ctx);
}

The activity will finish when the CompletionStage is completed.

Other considerations

It should be noted that Azure Functions for Java doesn't yet support non-blocking functions: https://github.com/Azure/azure-functions-java-worker/issues/244. Support for non-blocking activity tasks likely won't benefit Azure Functions users until the Java worker for Azure Functions adds async function support. Unfortunately, there's no indication about if or when this support will be added.

从 TaskActivity 接口开始，追踪其同步 run 结果的使用方式。在确定实现范围之前，审查拟议的 AsyncTaskActivity 契约以及链接的 Azure Functions 限制。完成的标准是：就 activity 的非阻塞完成方式及其与现有 activity 执行流程的集成达成一致方案。