Introduce nested expansion for resource group name

What does this MR do and why?

Introduce nested expansion for resource group name

Following the suggested code update in the linked issue, change where the Resource Group names are generated to expand all nested variables found within the string.

Introduce a feature flag for the capability and add testing with both states of the flag.

GitLab Issue: Nested variables in build:resource_group are no... (#361438 - closed)

MR acceptance checklist

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Before	After

How to set up and validate locally

• Create a CI YAML file with two variables and a build with a resource group.
  • The first variable should contain text: var: "blah"
  • The second variable should reference the contents of the first: foo: ${var}_text
  • The resource group should reference the contents of the second: resource_group: ${foo}_group
• Kick off the pipeline, see that only one layer is expanded
  • The job should refer to the resource group as ${var}_text_group
• Enable the feature flag
• Restart the pipeline and see that all nested references are expanded
  • The job should refer to the resource group as blah_text_group