Bug Fix - support environment variable for TF_VAR_private_key and TF_VAR_private_key_password

[Boulet-]

When a Default value is defined, the DefaultFunc is ignored/not called. Use the second parameter of DefaultFunc to put the default value.

To reproduce the issue, try to do a plan using TF_VAR_private_key or OCI_PRIVATE_KEY instead of TF_VAR_private_key_path

[oracle-contributor-agreement]

Thank you for your pull request and welcome to our community! To contribute, please sign the Oracle Contributor Agreement (OCA).
The following contributors of this PR have not signed the OCA:

• PR author: @Boulet-
• jeremy.albinet@brainboard.co

To sign the OCA, please create an Oracle account and sign the OCA in Oracle's Contributor Agreement Application.

When signing the OCA, please provide your GitHub username. After signing the OCA and getting an OCA approval from Oracle, this PR will be automatically updated.

If you are an Oracle employee, please make sure that you are a member of the main Oracle GitHub organization, and your membership in this organization is public.

[Boulet-]

@jotruon Any chance to have review for this one?
Thanks in advance

[jotruon]

assign reviewer and testing for this PR

[Boulet-]

assign reviewer and testing for this PR

I don't think I'm allowed to do it or I didn't find how to do it (no button under/near reviewers)

[tf-oci-pub]

Thank you for your valuable contribution. We greatly appreciate your efforts in submitting this pull request. However, I regret to inform you that we are unable to merge it directly on GitHub at this time.

Our internal policy requires that all pull requests undergo thorough local testing and review before they can be merged into the main codebase. This process ensures the quality and stability of Terraform-Provider-OCI.

We understand that this may cause some inconvenience, but please rest assured that your contribution is highly valued. Our team will carefully review and test your changes locally to ensure they meet our standards.

We appreciate your understanding and patience in this matter. If you have any questions or need further assistance, please don't hesitate to reach out. Thank you once again for your contribution.