Implement Signal Propagation Configuration Option

[yottahmd]

Overview
Add a configuration option DAGU_SIGNAL_PROPAGATION to enable signal propagation from the Dagu server/scheduler to running DAG processes.

Configuration Syntax in admin.yaml

signalHandling:
  enablePropagation: true

environment variable

DAGU_SIGNAL_PROPAGATION=1

Additional Context
https://discord.com/channels/1095289480774172772/1095291629344149544/1241031496698040392