feat: Add '--var-file' flag to iac test for loading external TF variable definition files [CFG-1663]

[ipapast]

• Follows CONTRIBUTING rules

What does this PR do?

Adds a --var-file flag to iac test for loading external Terraform variable definition files. See commit messages below for details.

Where should the reviewer start?

Follow the commits in order:

1. chore: add 'var-file' flag option to iac test command

This commit adds the 'var-file' flag as an available command under 'iac test'. CFG-1663

2. chore: Throw errors if 'var-file' flag is misused.

This commit adds checks around the 'var-file' flag usage: - If the user does not have the 'iacTerraformVarSupport' feature flag, we will throw an error. - If the user provides a non-existent path, we will throw an error. CFG-1663

3. feat: Load TF variable definitions files via --var-file

This commit adds the ability to load an external file by using 'var-file' flag:

• The user can point to a single filepath, relative to the 'pathToScan'
• The filepath will be checked for existence, and any vars will be de-referenced into the context of the 'pathToScan' directory only.

How should this be manually tested?

assign yourself the iacTerraformVarSupport flag.

1. Run: snyk-dev iac test test/fixtures/iac/terraform/var_deref/nested_var_deref and check the issues.
2. Run snyk-dev iac test test/fixtures/iac/terraform/var_deref/nested_var_deref --var-file=test/fixtures/iac/terraform/vars.tf and check that the sg_open_ssh.tf has one issue more that is found due to the allow_ssh_external_var_file var.

"Bad case" scenarios:

1. un-assign yourself the iacTerraformVarSupport flag and try to run scan (2) from above
2. assign yourself the iacTerraformVarSupport flag and point to a non-existing path or directory and see the relevant error
3. point to an invalid hcl (or other format) file and see the "failed to parse error" for that file.

I also ran checks with other flags, such as detection-depth, sarif, json.

What are the relevant tickets?

CFG-1663

Screenshots

Without the flag:

With the use of the flag (and finding an issue extra for the test/fixtures/iac/terraform/var_deref/nested_var_deref/sg_open_ssh.tf, when sharing the variable context of the external definitions file):

[ofekatr]

Spectacular work @ipapast,
There are a few nit comment with suggestions and questions, but none are blocking.
Feel free not to reply to suggestions.
LGTM 💯