chore: Cleaning up OIDC Work

[yhakbar]

Description

Cleans up the work done to introduce --terragrunt-iam-web-identity-token.

Changes

• Adds docs for the --terragrunt-iam-web-identity-token flag.
• Fixes tests for the flag so that they run in CircleCI.
• Integrates role assumption into the --terragrunt-auth-provider-cmd flag.

TODOs

Read the Gruntwork contribution guidelines.

• Update the docs.
• Run the relevant tests successfully, including pre-commit checks.
• Ensure any 3rd party code adheres with our license policy or delete this line if its not applicable.
• Include release notes. If this PR is backward incompatible, include a migration guide.

Release Notes (draft)

• Updated --terragrunt-iam-web-identity-token.
• Updated --terragrunt-auth-provider-cmd.