Add `cron` based policies

[TRAD-Anthony-CKO]

Is your feature request related to a problem? Please describe.
I wanted to write a policy that auto-close some PRs after a certain time (5 Days for example). I didn't find anything about supporting scheduled runs in policies. Is that supported somewhere? Any example I can use as a baseline?

Describe the solution you'd like
Here's an example policy I tried to write:

# -*- mode: yaml -*-

manifest:
  version: 1.0

automations:
  auto_close_old_prs:
    schedule:
      cron: "0 0 * * *"
    if:
      - {{ pull_request.age_days > 10 }}
      - {{ 'auto-close' in pull_request.labels }}
    run:
      - action: close-pr@v1
        args:
          reason: "This PR has been automatically closed as it has been open for more than 10 days without being merged or updated and is tagged with 'auto-close'. Please feel free to reopen or create a new PR if the changes are still relevant."

pull_request:
  age_days: {{ now() | date:'Y-m-d' | diff(pull_request.created_at | date:'Y-m-d') | days }}
  labels: {{ pull_request.labels | map(attribute='name') }}  

[PavelLinearB]

Hi @TRAD-Anthony-CKO, thanks for raising this issue
We are planning to add a cron trigger. But I'd like to make sure that our planned feature will address this use case
I will update here on the details and ETA

[TRAD-Anthony-CKO]

Hi @PavelLinearB
Any updates on this or ETA you can share? Thanks.

[vim-zz]

Hi @TRAD-Anthony-CKO I can confirm that this has been prioritized. However, I cannot provide a specific date at this time.

[brianespinosa]

@vim-zz is there any publicly accessible roadmap you could direct me to that would give indication on where this is at in terms of priority? Given that gitStream can deployed from one repository on an organization, a cron trigger would be incredibly helpful for those of us maintaining multiple large orgs, especially across an enterprise.