Add Nexus worker service for server-to-worker commands (#708)

## Summary

Defines a Nexus service for server-to-worker communication, starting
with activity cancellation support.

## Design Decision

We chose a **generic command API** (`ExecuteCommandsRequest` with
`oneof` command types) instead of a cancel-specific API. This allows a
future optimization to batch multiple commands (cancel, pause, etc) in a
single request and deliver to a worker in one RPC.

## Files

- `temporal/api/nexusservices/workerservice/v1/request_response.proto` -
request response definitions
- `nexus-rpc/temporal-proto-models-nexusrpc.yaml` - Nexus service
definition

## Related

- [Server PR](temporalio/temporal#9233)

---------

Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: rkannan82