[Feature Request] Nexus handler definitions

[chris-olszewski]

Rust SDK can invoke Nexus operations from workflows but cannot implement/handle them. Need Nexus service definition, handler registration on workers, and operation context.

[sachinsharma3191]

Design Proposal: Nexus Handler Definitions

To cleanly integrate Nexus Operations, we should provide an idiomatic definition structure that natively bonds with the existing Temporal worker infrastructure.

1. Handler Trait & Registry

• Defines an OperationHandler trait that expects a NexusOperationRequest and returns a NexusOperationResponse (which maps to either a synchronous successful/failed outcome payload, or an Async return token bridging into a background process/workflow).
• Introduce an OperationRegistry builder on the WorkerOptions similar to how Activities and Workflows are dynamically registered today.

2. Core Dispatch Hooks

• The Core task polling infrastructure must accommodate processing PollNexusTaskQueue tasks.
• When an operation is popped from the task queue, Core translates it into a standard activation job directed to the language layer.
• Once the user's OperationHandler future resolves, Core receives an OperationCompletion event to report back.

[sachinsharma3191]

@chris-olszewski, whenever you have a moment, could you take a look at the design for this? Also, let me know if you’re already tackling this so we don't double up on effort.