Conditional step execution

[muenchdo]

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

Add the possibility to add conditions for promotion step executions, similar to what is available in GitHub Actions. The expressions added with v1.1 would make this very useful.

Motivation

Right now the main use case I have in mind is being able to run something like a "finalizer" to send a Slack notification when the promotion fails, but I feel like this would be useful in more cases.

Suggested Implementation

GitHub Actions seems to be a good inspiration here.

steps:
- uses: foo
  alias: fooStep
  config:
    option1: value1
    option2: value2
- uses: bar
  if: ${{ len(outputs.fooStep.someOutput) > 0 }}
  config:
    option1: value1
    option2: value2

For my finalizer use case, we would need some special expressions such as $always().

Looking at #3124, it seems there are technical limitations that make this difficult, but I do think this (especially the finalizer use case) is less of an edge case than loops.

[krancour]

You're on the same page as me. I think if is fairly easy to do. As for "always," I think we could potentially add something like a "failure policy" that can be configured to allow a process (or designated steps of a process) to continue after a failure. This bit is harder and would require some design work. If we split "always" into a separate issue, "if" on its own can probably get done faster.