Adds the specified employees to a payroll run and streams progress updates to the client using Server-Sent Events (SSE).

The response is streamed as 'text/event-stream', allowing clients to receive real-time
progress updates. This endpoint is intended for use with clients that support SSE. Response buffering is
disabled when possible to ensure timely delivery of events.

         Progress event payload:
         1. Status enum values - Invalid (0), NotStarted (1), Started (2) , CancelRequested (3), Cancelled (4), Finished (5)
         2. Progress (int) - percentage of completion (0-100)
         3. Errors (list) - each item represents blocking error (Type = HardStopError(2)) or a warning (Type = SoftWarning (1))
        
         Note: HardStopError can stop payroll and it won't be possible to call AddEmployees API enpoint again.