feat: add actor metadata to Promotion context

[keithfz]

As a prerequisite to adopting Kargo, we want to be able to include information about the user triggering the promotion in our promotion templates. Specifically so the authenticated user triggering a promotion is tracked in the Git history of our GitOps repo.

To implement this, I followed the same pattern & logic used by the Created By column in the frontend where we parse the kargo.akuity.io/create-actor annotation. If it doesn't exist, we return N/A. This gets added to the promotion context, and can then be exposed in an expression.

kargo/ui/src/features/stage/promotions.tsx

Lines 150 to 155 in 29e6fb8

	title: 'Created By',
	render: (_, promotion) => {
	const annotation = promotion.metadata?.annotations['kargo.akuity.io/create-actor'];
	const email = annotation ? annotation.split(':')[1] : 'N/A';
	return email || annotation;

For our use case around auditability in the commit history, this suffices.

Relates to issue: #3081

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[codecov]

Codecov Report

Attention: Patch coverage is 93.75000% with 1 line in your changes missing coverage. Please review.

Project coverage is 52.85%. Comparing base (abea4d1) to head (5fac7f4).

Files with missing lines	Patch %	Lines
internal/controller/promotions/promotions.go	90.90%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3592      +/-   ##
==========================================
+ Coverage   52.82%   52.85%   +0.02%     
==========================================
  Files         299      299              
  Lines       26892    26908      +16     
==========================================
+ Hits        14206    14221      +15     
- Misses      11924    11925       +1     
  Partials      762      762              

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[hiddeco]

Made some changes to facilitate further growth better, and to simplify potential comparison checks people may want to do at some point:

• Renamed and moved the field from ctx.creator to ctx.meta.promotion.actor.
• Dropped the N/A in case none is found, this makes it much simpler to do e.g. != "" type of checks.
• Slightly modified the extraction logic to incorporate a potential admin value that might be set.

[keithfz]

Awesome! Changes make sense to me.

Should we also update the docs accordingly for the new path you’ve added?

kargo/docs/docs/50-user-guide/60-reference-docs/40-expressions.md

Line 146 in 8d60225

| `ctx` | `object` | `string` fields `project`, `stage`, `promotion`, and `creator` provide convenient access to details of a `Promotion`. |

[keithfz]

Should update this to new field?

[hiddeco]

Yes, this still needs to be updated, but I am struggling to find the best way to address this as we seem to be running into the limitations of using tables.