feat(api/controller): Promotion directive steps

[hiddeco]

This introduces a PromotionTemplate within Stage resources, allowing someone to define a sequence of steps (directives) that should be run to promote Freight to a Stage using a Promotion.

---
apiVersion: kargo.akuity.io/v1alpha1
kind: Stage
metadata:
  name: example
spec:
  promotionTemplate:
    spec:
      steps:
      - step: <directive>
      - step: <directive>

When this field is defined for a Stage, any previous "Promotion mechanisms" are ignored for newly created Promotions.

---

Note: While this pull request makes the feature work from end to end, it still has quite some rough edges, including (nonfunctioning) work directory persistence, result and queue behavior, Stage health checks, etc.

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[codecov]

Codecov Report

Attention: Patch coverage is 47.61905% with 55 lines in your changes missing coverage. Please review.

Project coverage is 49.54%. Comparing base (fbd3953) to head (abf22ae).
Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
internal/controller/promotions/promotions.go	27.27%	24 Missing ⚠️
internal/directives/simple_engine.go	77.77%	5 Missing and 5 partials ⚠️
api/v1alpha1/stage_types.go	0.00%	8 Missing ⚠️
api/v1alpha1/promotion_types.go	0.00%	7 Missing ⚠️
internal/directives/git_clone_directive.go	0.00%	3 Missing ⚠️
internal/kargo/kargo.go	0.00%	1 Missing and 1 partial ⚠️
internal/controller/stages/stages.go	0.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2539      +/-   ##
==========================================
- Coverage   49.60%   49.54%   -0.07%     
==========================================
  Files         275      276       +1     
  Lines       19690    19761      +71     
==========================================
+ Hits         9767     9790      +23     
- Misses       9318     9363      +45     
- Partials      605      608       +3     

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[krancour]

+1 to PromotionContext