Add auth support for cgr.dev (Chainguard Images)

[tstromberg]

Obsoletes #238

The Chainguard Images registry (cgr.dev) require a temporary token to be acquired for authentication. Without adding cgr.dev to _WWW_AUTH, the test fails with:

WARNING: Download from https://cgr.dev/v2/chainguard/static/manifests/latest failed: class com.google.devtools.build.lib.bazel.repository.downloader.UnrecoverableHttpException GET returned 401 Unauthorized
ERROR: An error occurred during the fetch of repository 'chainguard_static':
   Traceback (most recent call last):
	File "/home/t/src/rules_oci/oci/private/pull.bzl", line 477, column 51, in _oci_alias_impl
		manifest, _ = downloader.download_manifest(rctx.attr.identifier, "mf.json")
	File "/home/t/src/rules_oci/oci/private/pull.bzl", line 280, column 74, in lambda
		download_manifest = lambda identifier, output: _download_manifest(rctx, state, identifier, output),
	File "/home/t/src/rules_oci/oci/private/pull.bzl", line 255, column 18, in _download_manifest
		_download(
	File "/home/t/src/rules_oci/oci/private/pull.bzl", line 223, column 23, in _download
		return download_fn(
	File "/home/t/src/rules_oci/oci/private/download.bzl", line 83, column 35, in _download
		command.extend(_auth_to_header(url, auth))
	File "/home/t/src/rules_oci/oci/private/download.bzl", line 9, column 24, in _auth_to_header
		if auth_val["type"] == "pattern":
Error: key "type" not found in dictionary