Add API to resolve manifest for current platform

Add open_image_this_platform() which resolves a manifest from an OCI
directory for the native platform (OS and architecture). This handles
manifest lists by peeling them one level to find platform-specific
manifests.

Returns ResolvedManifest containing:
- The resolved manifest and its digest
- The source image index and digest if resolution went through a
  manifest list

Assisted-by: OpenCode (claude-opus-4-5)

Author: cgwalters

Cargo.toml

@@ -11,6 +11,7 @@ keywords = ["oci", "opencontainers", "docker", "podman", "containers"]
 camino = "1.0.4"
 chrono = "0.4.19"
 cap-std-ext = "5.0"
+either = "1"
 flate2 = { features = ["zlib"], default-features = false, version = "1.0.20" }
 hex = "0.4.3"
 openssl = "0.10.33"