Add custom tf var from vars and secrets capability

[jaredfholgate]

Description

This PR adds the capability to send and vars or secrets that start with TF_VAR_ to the test run as env vars, so that they can be leveraged in the run.

A use case for this is testing with GitHub or Azure DevOps where we need to supply PAT and org name.

Here is an example test run leveraging this code: https://github.com/Azure/terraform-azurerm-avm-ptn-cicd-agents-and-runners/actions/runs/10199435253/job/28217447877

Type of Change

• [ ] Non-module change (e.g. CI/CD, documentation, etc.)
• [ ] Azure Verified Module updates:
  • [ ] Bugfix containing backwards compatible bug fixes
  • [ ] Someone has opened a bug report issue, and I have included "Closes #{bug_report_issue_number}" in the PR description.
  • [ ] The bug was found by the module author, and no one has opened an issue to report it yet.
  • [x] Feature update backwards compatible feature updates.
  • [ ] Breaking changes.
  • [ ] Update to documentation

Checklist

• [x] I'm sure there are no other open Pull Requests for the same update/change
• [x] My corresponding pipelines / checks run clean and green without any errors or warnings
• [x] I did run all pre-commit checks

[matt-FFFFFF]

LGTM