[DO NOT MERGE] Generate nexus service definitions

[spkane31]

What changed?
Adds Nexus service definition generation for WorkflowService (workflowservice/v1/workflowservicenexus/service_nexus.pb.go) driven by nexus-rpc-gen reading the github.com/temporalio/api:nexus/temporal-proto-models-nexusrpc.yaml files which defines SignalWithStartWorkflowExecution.

Add a nexus-gen Makefile target for generation and wiring into the grpc target.

Why?
Temporal needs to expose WorkflowService RPCs as Nexus operations (those marked as exposed).

How did you test it?
Server PR: insert here

Potential risks

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