Add `remotes/certutil` functions for loading `ca.crt`, `client.cert`, and `client.key` into `tls.Config` from a directory like `/etc/docker/certs.d/<hostname>.
See https://docs.docker.com/engine/security/certificates/ .
Client applications including CRI plugin are expected to configure the resolver using these functions.
As an example, the `ctr` tool is extended to support `ctr images pull --certs-dir=/etc/docker/certs.d example.com/foo/bar:baz`.
Tested with Harbor 1.8.
Signed-off-by: Akihiro Suda <akihiro.suda.cz@hco.ntt.co.jp>
