cmd: add `kes ls` command

[aead]

This commit adds the kes ls command that lists keys, policies and identities.

Usage:
    kes ls [-a KEY] [-k] [--json] [-i] [-p] [-s HOST[:PORT]] [PREFIX]

Options:
    -a, --api-key KEY           API key to authenticate to the KES server.
                                Defaults to $MINIO_KES_API_KEY.
    -s, --server HOST[:PORT]    Use the server HOST[:PORT] instead of
                                $MINIO_KES_SERVER.
        --json                  Print output in JSON format.
    -i, --identity              List identities.
    -p, --policy                List policy names.
    -k, --insecure              Skip server certificate verification.

This command replaces kes key ls, kes policy ls and kes identity ls mid-term.