feat: add --var-file support

[YairZ101]

• Follows CONTRIBUTING rules

What does this PR do?

This PR adds support for the --var-file flag in the new IaC testing flow.

How should this be manually tested?

1. Build a local binary of the following snyk-iac-test branch
2. Files to test this new functionality are available here
3. cd to the directory linked above and run the following commands:
4. SNYK_IAC_POLICY_ENGINE_PATH={local_snyk-iac-test_exe_path} snyk-dev iac test terraform.tf --experimental --var-file=random and get an error for non-existent vars file
5. SNYK_IAC_POLICY_ENGINE_PATH={local_snyk-iac-test_exe_path} snyk-dev iac test terraform.tf --experimental --var-file=../valid/terraform.tf and get an error for an invalid vars file
6. SNYK_IAC_POLICY_ENGINE_PATH={local_snyk-iac-test_exe_path} snyk-dev iac test terraform.tf --experimental --var-file=vars.tfvars and get test results

What are the relevant tickets?

https://snyksec.atlassian.net/jira/software/c/projects/CFG/boards/301?modal=detail&selectedIssue=CFG-2113

[github-actions]

	Warnings
⚠️	You've modified files in src/ directory, but haven't updated anything in test folder. Is there something that could be tested?

Generated by 🚫 dangerJS against 537372d

[ipapast]

tested against all scenarios and also *auto.tfvars files, works great 🌟