How to express "Job Update" in portable Beam

[kennknowles]

Based in the discussion here https://docs.google.com/a/google.com/document/d/1xOaEEJrMmiSHprd-WiYABegfT129qqF-idUBINjxz8s/edit?disco=AAAAB9MqC-A

For portable beam, how can user express intent to update the a running job.

Imported from Jira BEAM-4289. Original Jira may contain additional context.
Reported by: angoenka.