Implement native OIDC-based authentication for Chainguard's container
registry, following the same pattern as the existing AWS ECR integration.
When registry is set to cgr.dev, the action automatically exchanges a
GitHub Actions OIDC token with Chainguard's STS endpoint for a short-lived
registry credential, removing the need for chainctl or long-lived pull tokens.
New inputs: chainguard (auto/true/false), chainguard-identity.
Signed-off-by: Augustus Nguyen <theflash28012002@gmail.com>
