Provide promotion trigger information in the promotion context

[schildwaechter]

Checklist

• I've searched the issue queue to verify this is not a duplicate feature request.
• I've pasted the output of kargo version, if applicable.
• I've pasted logs, if applicable.

Proposed Feature

Provide all the events associated to a promotion in the context, in particular the trigger.
Currently, the events view shows lines such as the following.

• Automatically promoted Freight from origin "Warehouse/XXXXX" for Stage "YYYYY"
• Promotion created for Stage "ZZZZZ" by "admin"

These should be available to re-use in the promotion steps, in particular for Git commit messages.

Motivation

This would allow better traceability of why Kargo is making changes.
This would allow to propagate reasons for changes to other systems, such as logging infrastructure or to include them in the git commit history.

Suggested Implementation

The ctx.promotion context could be extended with the message that's sent to the events view. Alternatively, the information on whether it's an automated or manual promotion and which user triggered it, could be added as individual properties.

[hiddeco]

Related to #3081

[schildwaechter]

With the implementation from #3592, the user triggering the promotion becomes available and in cases of auto-promotion it's empty, so that's very nice already.
However, I would still suggest to add an attribute explicitly stating it's an automatic promotion for the purposes of tracebility.
Might also be worth to make the freight's alias available.

[krancour]

Might also be worth to make the freight's alias available

Those mutable, so they're not the best thing to use in any sort of historical record.