feat: Add credential helper support to oci-distribution

- Move Docker config parsing and credential helper execution from krust
- Add automatic auth resolution methods (*_auto) to the client
- Support standard Docker config locations and environment variables
- Add comprehensive tests and examples
- Simplify krust to use the new credential helper functionality

src/auth/simple.rs

@@ -0,0 +1,11 @@
+//! Simple authentication wrapper using oci-distribution's credential helper
+
+use anyhow::Result;
+use oci_distribution::credential_helper::resolve_docker_auth;
+use oci_distribution::secrets::RegistryAuth;
+
+/// Resolve authentication for a given resource using Docker config and credential helpers
+pub fn resolve_auth(resource: &str) -> Result<RegistryAuth> {
+    resolve_docker_auth(resource)
+        .map_err(|e| anyhow::anyhow!("Failed to resolve auth: {}", e))
+}