Add support for variable placeholders in Manifests (compliment to values placeholders)

[nicc777]

Brief Description of the Enhancement

Extend the feature of supporting values in manifests and support for variable substitution based on prior processed manifests (implied dependency, in this case)

Describe the benefit of this proposed enhancement

Allows the implementation of some manifests that set some variable values to re-use those values in other manifests.

Code examples, pseudo code or any other technical description of the proposal

General parsing rule:

• The final result must be a str type value. Therefore, if the following conversions should be done:
  • For values of type dict or list: convert to a string with the JSON representation
  • For encoded strings, decode the string
  • For int or Decimal values, convert to string
  • For other objects, call the __str__ method (use str(value))
  • None will be converted to an empty string
• If the variable is not found, the final result will be an empty string

Basic example:

---
kind: ShellScript
version: v1
metadata:
  name: cli-get-aws-secret-sandbox
  skipDeleteAll: true
  environments:
  - sandbox
spec:
  source:
    type: filePath
    path: '/opt/get-aws-sandbox-secret.sh'
---
kind: AwsBoto3Session
version: v1
metadata:
  name: aws-boto3-session-v1-aws-session-access-key-sandbox
  dependencies:
    apply: cli-get-aws-secret-sandbox
  environments:
  - sandbox
  executeOnlyOnceOnApply: true
  skipDeleteAll: true
spec:
  awsAccessKeyId: ABCDEFGHIJKLMNOPQRSTUVWXYZ
  awsRegion: eu-central-1
  awsSecretAccessKey:
    source: '{{ .Variables.ShellScript:cli-get-aws-secret-sandbox:sandbox:STDOUT }}'