Make variables in promotiontask required or not

[fernandomendieta-facephi]

Checklist

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

kargo version -> 1.7.4

Proposed Feature

Optional Variable in promotion task

Motivation

While using the same promotion task for different environments and using variable to pass them throught setValues. in helm-template step, easy example here using a variable called variablename:

Example error:
admission webhook "promotion.kargo.akuity.io" denied the request: failed to inflate Promotion steps: inflate tasks steps for task "promote" ("promote"): missing value for variable "variablename"

vars: 
- name: variablename
 
  - uses: helm-template
    config:
      path: ${{ vars.srcPath }}/stages/app
      buildDependencies: true
      namespace: ${{ vars.appNamespace }}
      valuesFiles:
      - ${{ vars.srcPath }}/stages/app/values.yaml
      setValues:
        - key: key1.key2.key3
          value: ${{ vars.variablename }}

Let's say the stage of dev passes that variable, so no error, but stg doesn't need that variable and to have the flow cleaner dont want to pass a dummy variable.

Example on how to pass variables from the stage dev that works fine :

apiVersion: kargo.akuity.io/v1alpha1
kind: Stage
metadata:
  name: dev
spec:
      ......
        vars:
        - name: variablename
          value: "value1"

Example on how to pass variables from the stage stg that returns an error when promoting :

apiVersion: kargo.akuity.io/v1alpha1
kind: Stage
metadata:
  name: stg
spec:
      ......
        vars:
        - name: variablename
          value: ""

So imagine we can put in the pomotiontask a variable for each variable that handled if the variable is required or not, example here following the previous:

vars: 
- name: variablename
 
  - uses: helm-template
    config:
      path: ${{ vars.srcPath }}/stages/app
      buildDependencies: true
      namespace: ${{ vars.appNamespace }}
      valuesFiles:
      - ${{ vars.srcPath }}/stages/app/values.yaml
      setValues:
        - key: key1.key2.key3
          value: ${{ vars.variablename }}
          required: false

So in that case we would not receive the error above

[snuggie12]

This really reduces places to use the ?? operator. Instead we have to make everything a ternary that checks for the string EMPTY.