Introduce dummy forecasting script

What does this MR do and why?

This MR is part of https://gitlab.com/gitlab-org/gitlab/-/issues/403630

Introduce dummy forecasting script

This script does nothing useful yet.
It's created for infrastructure team to rely
on stable entrypoint and test that
script dependencies are properly
installed.

How to set up and validate locally

1. Add python 3.8.16 to .tool-versions. Run asdf install
2. Reload terminal. Run pip install prophet
3. Run python scripts/forecast_timeseries.py.
4. You should see {'ds': [], 'y': [42]}

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #403630