Make `--setup` work with `--num-procs`

[hoxbro]

Resolves #6891

Postpone the execution to the start the server.

It needs a bit more thought than what I have given it. This is mostly a POC right now.

# app.py
import panel as pn

pn.pane.Markdown("Hello World").servable()

# background_job.py
from datetime import datetime
import os
import panel as pn

current_time = datetime.now()


def task() -> None:
    """Refresh the data in the database."""
    print(f"Ran task at {datetime.now()} (PID {os.getpid()})")


pn.state.schedule_task(name="load_data", callback=task, at=current_time, period="5s")

Command: panel serve app.py --setup background_job.py --num-procs 2

https://github.com/holoviz/panel/assets/19758978/035e6f2d-269d-49a2-a46a-38669f793a92