Add missing auth validation for OCI Backend

[robo-cap]

Add missing validation for OKEWorkloadIdentity auth in the oci backend.

Resolves error below when oci backend auth = "OKEWorkloadIdentity".

 auth must be one of 'ApiKey' or 'InstancePrincipal' or
│ 'InstancePrincipalWithCerts' or 'SecurityToken' or 'ResourcePrincipal' or
│ 'OKEWorkloadIdentity'

Fixes #

Target Release

1.15.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[hashicorp-cla-app]

All committers have signed the CLA.

[github-actions]

Changelog Warning

Currently this PR would target a v1.15 release. Please add a changelog entry for in the .changes/v1.15 folder, or discuss which release you'd like to target with your reviewer. If you believe this change does not need a changelog entry, please add the 'no-changelog-needed' label.

[crw]

Thanks for this submission, I will raise it in triage.