feat: keep track of promotion state in promotion status

[krancour]

@hiddeco it is best for you to review this one.

I know we had several offline conversations about whether this was worth avoiding or something we'd be forced to do.

Waiting for a PR to merge or close is the thing I ended up really needing this for.

Going back and looking at legacy code, even then, we never really had a way of doing this without storing the PR number in promotion status' metadata field.

fwiw, this is the last thing standing in the way of all of these directive-based examples passing:

https://github.com/krancour/kargo-examples/tree/for-v0.9.0

If you are generally unopposed to this, but wish to change some implementation details, please feel free to amend this PR to maximize the possibility that we cut a release candidate early in the day tomorrow.

Edit: A tangential benefit of this is it provides a little more visibility into what step is holding up a long-running promotion.

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[codecov]

Codecov Report

Attention: Patch coverage is 26.37363% with 67 lines in your changes missing coverage. Please review.

Project coverage is 51.05%. Comparing base (022c431) to head (e98b457).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/directives/git_pr_waiter.go	6.66%	28 Missing ⚠️
internal/controller/promotions/promotions.go	0.00%	16 Missing ⚠️
internal/directives/simple_engine.go	66.66%	10 Missing and 1 partial ⚠️
api/v1alpha1/promotion_types.go	0.00%	7 Missing ⚠️
internal/directives/state.go	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2604      +/-   ##
==========================================
- Coverage   51.09%   51.05%   -0.04%     
==========================================
  Files         282      282              
  Lines       20961    21007      +46     
==========================================
+ Hits        10709    10726      +17     
- Misses       9584     9614      +30     
+ Partials      668      667       -1     

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[hiddeco]

No real objection from me. I wonder if in the future we continue to want to persist all state to the object (and not e.g. a sub-set which has to survive), but I think this concern can be addressed later.