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

examples/auto_auth_demo.rs

@@ -0,0 +1,71 @@
+//! Example demonstrating automatic authentication with oci-distribution
+//!
+//! This example shows how to use the automatic auth methods that resolve
+//! credentials from Docker config files and credential helpers.
+
+use anyhow::Result;
+use oci_distribution::{client::Client, Reference};
+
+#[tokio::main]
+async fn main() -> Result<()> {
+    // Initialize logging
+    tracing_subscriber::fmt::init();
+
+    // Create a client
+    let client = Client::default();
+
+    // Example 1: Pull a public image (anonymous auth)
+    println!("Pulling public image alpine:latest...");
+    let reference: Reference = "docker.io/library/alpine:latest".parse()?;
+
+    match client.pull_manifest_auto(&reference).await {
+        Ok((manifest, digest)) => {
+            println!("✓ Successfully pulled manifest");
+            println!("  Digest: {}", digest);
+            match manifest {
+                oci_distribution::manifest::OciManifest::Image(img) => {
+                    println!("  Type: Single platform image");
+                    println!("  Config: {}", img.config.digest);
+                }
+                oci_distribution::manifest::OciManifest::ImageIndex(idx) => {
+                    println!("  Type: Multi-platform image");
+                    println!("  Platforms: {}", idx.manifests.len());
+                }
+            }
+        }
+        Err(e) => {
+            println!("✗ Failed to pull: {}", e);
+        }
+    }
+
+    // Example 2: Get platforms for an image
+    println!("\nDetecting platforms for rust:alpine...");
+    let rust_ref: Reference = "docker.io/library/rust:alpine".parse()?;
+
+    match client.get_image_platforms_auto(&rust_ref).await {
+        Ok(platforms) => {
+            println!("✓ Found {} platforms:", platforms.len());
+            for (os, arch) in platforms {
+                println!("  - {}/{}", os, arch);
+            }
+        }
+        Err(e) => {
+            println!("✗ Failed to get platforms: {}", e);
+        }
+    }
+
+    // Example 3: Private registry (requires auth)
+    println!("\nNote: To test with a private registry, ensure you have credentials in:");
+    println!("  - ~/.docker/config.json");
+    println!("  - Or DOCKER_CONFIG environment variable");
+    println!("  - Or using a credential helper");
+
+    // Uncomment to test with a private registry:
+    // let private_ref: Reference = "ghcr.io/your-username/your-image:latest".parse()?;
+    // match client.pull_manifest_auto(&private_ref).await {
+    //     Ok((_, digest)) => println!("✓ Successfully authenticated and pulled private image: {}", digest),
+    //     Err(e) => println!("✗ Failed to pull private image: {}", e),
+    // }
+
+    Ok(())
+}