feat: adds @cron task decorator

[mikeshultz]

What I did

Adds the @cron task decorator so we can execute arbitrary tasks according to a crontab schedule.

How I did it

Adds an event loop to the runner that checks if cron tasks should be executes. If true, it will kiq the tasks off.

How to verify it

@app.cron("*/5 * * * *")
def every_five_minutes():
    return {"crontime": datetime.utcnow()}

Checklist

• [x] Passes all linting checks (pre-commit and CI jobs)
• [ ] New test cases have been added and are passing
• [x] Documentation has been updated
• [x] PR title follows Conventional Commit standard (will be automatically included in the changelog)

[fubuloubu]

~~Upgraded this, but need to figure out why crons aren't triggering~~

---

Edit: figured out it was due to an internal failure 7a93ab3, which weren't getting displayed properly b77e718

[fubuloubu]

p.s. this website had some good test cases: https://crontab.guru/

[fubuloubu]

closed in favor of #230