feat(echo): add a configurable experiment duration to the dev Echo app

[yasithdev]

Summary

• Added an Experiment_Duration input (INTEGER, default 60s) to the Echo application interface, ordered before the message.
• Routed the Echo deployment through a new conf/slurm/echo_sleep wrapper (sleep <duration>; echo <message>) instead of /bin/echo, so the job sleeps for the given duration before echoing. The wrapper is baked into the SLURM image.

Test plan

• Launch Echo with duration 30/60 → generated submit script is /usr/local/bin/echo_sleep <duration> <message>; the job sleeps, then writes Echo.stdout = the message.
• Experiment runs through the full pipeline (env setup → submit → monitor → data staging); the longer runtime is observable in the experiment progress view.