Improved orchestrator and activity input/output handling

Background

The current TaskOrchestration and TaskActivity interfaces require implementations to use the context object to receive inputs. Consider the following example (copied from the integration tests):

final String orchestratorName = "SingleActivity";
final String activityName = "Echo";
final String input = Instant.now().toString();
DurableTaskGrpcWorker worker = this.createWorkerBuilder()
    .addOrchestrator(orchestratorName, ctx -> {
        String activityInput = ctx.getInput(String.class);
        String output = ctx.callActivity(activityName, activityInput, String.class).get();
        ctx.complete(output);
    })
    .addActivity(activityName, ctx -> {
        return String.format("Hello, %s!", ctx.getInput(String.class));
    })
    .buildAndStart();

This design, while it works, has a few issues:

1. The fetching of inputs is not type-safe because the call to ctx.getInput(String.class) could fail if the input isn't actually a string.
2. The call to ctx.getInput() isn't intuitive, making the programming model harder for developers to learn.
3. The call to ctx.complete(output) is both unintuitive and inconsistent with how activity functions are defined.

This issue tracks improving the programming model to make processing inputs and outputs more intuitive and type-safe.

Proposal

The proposal is to change these interface definitions so that a developer could write the following, simpler orchestration and activity implementations:

final String orchestratorName = "SingleActivity";
final String activityName = "Echo";
final String input = Instant.now().toString();
DurableTaskGrpcWorker worker = this.createWorkerBuilder()
    .addOrchestrator(orchestratorName, (ctx, activityInput) -> {
        return ctx.callActivity(activityName, activityInput, String.class).get();
    })
    .addActivity(activityName, (ctx, name) -> {
        return String.format("Hello, %s!", name);
    })
    .buildAndStart();

The differences are:

• The orchestrator and activity functions have the input passed to them explicitly.
• The orchestrator can use the return value to set the output.

This results in a simpler, more intuitive programming model and is also consistent with the proposed C# programming model.

Start by reviewing the TaskOrchestration and TaskActivity interfaces and the integration-test examples referenced in the issue. Trace how inputs and outputs currently flow through the context, then update the programming model so implementations receive typed inputs and return outputs as shown; the integration tests should demonstrate the proposed usage.