podman load: support downloading files

Support downloading files, for instance via
`podman load -i server.com/image.tar`.  The specified URL is downloaded
in the frontend and stored as a temp file that gets passed down to the
backend.

Also vendor in c/common@main to use the new `pkg/download`.

Fixes: #11970
Signed-off-by: Valentin Rothberg <rothberg@redhat.com>

vendor/github.com/containers/common/pkg/download/download.go

@@ -0,0 +1,31 @@
+package download
+
+import (
+	"fmt"
+	"io"
+	"io/ioutil"
+	"net/http"
+)
+
+// FromURL downloads the specified source to a file in tmpdir (OS defaults if
+// empty).
+func FromURL(tmpdir, source string) (string, error) {
+	tmp, err := ioutil.TempFile(tmpdir, "")
+	if err != nil {
+		return "", fmt.Errorf("creating temporary download file: %w", err)
+	}
+	defer tmp.Close()
+
+	response, err := http.Get(source) // nolint:noctx
+	if err != nil {
+		return "", fmt.Errorf("downloading %s: %w", source, err)
+	}
+	defer response.Body.Close()
+
+	_, err = io.Copy(tmp, response.Body)
+	if err != nil {
+		return "", fmt.Errorf("copying %s to %s: %w", source, tmp.Name(), err)
+	}
+
+	return tmp.Name(), nil
+}