[Compute Separation] PROTO: Add host_configuration_json to StartStream

[brettsam]

Add new fields to the gRPC protocol for separated compute. These fields are optional and ignored when external worker mode is disabled, so existing workers are unaffected.

Dependencies: None (can be done in parallel with M2)

Proto Changes

message StartStream {
  string worker_id = 2;
  string host_configuration_json = 3;  // Raw host.json content from the worker's filesystem
}

Acceptance Criteria

• StartStream has host_configuration_json field
• Proto compiles and generated C# code is updated
• Existing workers that don't send the new field are unaffected (field defaults to empty string)
• Build succeeds, existing tests pass